Does anyone know why the "whipping post/scapegoat" child so often becomes the narcissist's caregiver? Even when they do not get on as well?

I've read a lot of posts from people in your situation and here's my take on it:

Your mother moved near you because you are the one she has designated throughout life to be the slave to her needs. The golden child is above all of that, your needs and desires are the very last in the family hierarchy.
When the abused child becomes a caregiver it is because they have been conditioned to appease, to avoid conflict, to place themselves last and have never had therapy to understand differently. Somewhere down inside there is still a desperate child longing to finally win the love and respect of the narc parent.

Let me tell you this: I distinctly remember being told OVER AND OVER again as a child that 'your mother is perfect. You never say anything bad about your mother, even if she is a drunken bum on the street. You tell everyone she is wonderful.' I was groomed from early childhood to idolize my mother and recognize how 'wonderful' and 'perfect' she was and to preach that word all over the world.

So I grew up feeling insecure. Imperfect. Never up to mother's standards. Always feeling the need to jump thru the fiery hoops to please the mother who would and could never BE pleased. To make mommy happy. To be the good girl who would make mommy proud. And that sort of brainwashing stays with a person throughout their life.

And, as an only child (lucky me), I was the only one around to take charge of my parents' lives as they aged and became ill. My dad was a great guy, but bullied and abused by my mother for 68 years. After he died 5 years ago next week, mother lost her main scapegoat and whipping post and turned all her anger and rage on ME.

So what I do is this: I set boundaries with my mother. I call her 6 evenings per week (she thinks I work full time, ha!) and my DH calls her the other day. We both go to visit her 1x per week for 20 minutes (it's a window visit right now at the Assisted Living Facility where she lives due to COVID19 restrictions) and that's it. I pay her bills, I run her life, I send emails make phone calls, arrange to have a tooth pulled, a doctor visit, order incontinence supplies, EVERYTHING. But on MY terms, from MY desk, at MY convenience. I make the rules and I stick to them like glue.

Otherwise, mother would invade what's left of my life with even MORE negative hostility, nastiness and BS in general.

Let your husband read these responses and then decide how to proceed with the situation accordingly.

The scapegoat child is groomed to serve the narcissistic parent. The child is made to feel he/she doesn't deserve boundaries and is easily used. If the child sooner or later recognizes how she's been wronged and stands up for herself, the narcissist suffers a narcissistic injury and lashes out to punish them. It's awfully hard on the child. Sometimes the punishment lasts a lifetime, but to me it was worth it.

You answered your own question. Your husband is humble. Narcissistic people pick out humble, empathetic people because no one else will put up with them. Read Boundaries by Henry Cloud.

As the daughter of a Narc mother, the only thing that has saved me from sinking deeper and deeper into the emotional mess that I was becoming (or perhaps, did become for awhile) was to go grey rock with her.

She doesn't even notice. I did 8 months of cancer TX last year and she never called or inquired of anyone how I was. (sick, very, very sick).

When I went to see her, after chemo was over and I had just begun to grow some hair, her first words to me were "Oh, and you used to be so pretty". What a backhanded compliment.

Last week was her 90th birthday. My YS asked us to write her a letter and include good memories and such that we had of growing up. I just. Couldn't.

So I gave her a generic card and called it good. I'm sure YS thinks I was being a prima donna, but I won't lie to mother and pretend I feel like everything is peachy. I wasn't mean, I never AM, but I also do not have to kowtow to her.

I need peace--for myself to heal from a horrible year of being sick, and for my own self protection.

Sad for Y with whom mother lives. My SIL says she and YB fight all the time and he gets really, really angry. They both wish they'd never moved her into their home. 22 years now.

I remain a classic 'overpleaser'. But I choose whom I care for.

Helen, as my therapist always said "why do you continue to hold any expectation that things will ever change?" Sounds harsh in writing, to be sure; but she said it in a completely supportive and gentle way (no criticism, just factually), and educated me on the nature of being the family 'scapegoat' or 'black-sheep', and she continued for years to have patience with me while I SLOWLY absorbed the teaching of that truth. I now fully accept that NOTHING I do will change their attitude or behavior toward me--because it never was about me--my existence (my role) serves their own neurotic needs and fears. This hurt like hell, but less and less as time goes on, and has allowed room for me to recognize that acceptance is freedom and space to (at 65) finally, really, learn who I truly am--including my own neurotic needs and fears, ha! But I will never project them onto another human; this knowledge brings me peace and comfort, and gives me new space to grow : ) I wish everybody here who shares their experiences here with being the scapegoat and dealing with the 'can-do-no-wrong' sibling peace and my support. (That sib, naturally, uses and abuses that role that is completely unearned; but that's her inventory, not mine!)

Plus what others have already posted, there is also the generational belief that daughters are often (usually!) the ones who are supposed to be the parent's caregiver.

You are their caregiver because you keep thinking your parent might see all the good things you are doing and then finally realize you are a good person and finally stop being so mean to you. It sadly does not change anything though. Many of us here have walked the same path and made various choices as a result.

I haven't read any of the other replies yet, but my answer is -- because, subconsciously, we believe that this ultimate giving of ourselves will prove that we are worthy of their love.
Living far away from Mother for over 30 years with one week visits once a year, and then taking her on 24/7/365, helped me to realize that her version of love and mine are very different. I understand now that she has narcissistic tendencies. Covert narcissistic personality. And that this was a learned behavior, so probably her mother taught her. Makes me sad, because this is normal to her. It took me moving 600 miles away to break the cycle. But still, my wounded child self occasionally seeks approval and empathy from someone incapable of genuinely reciprocating.

I think another consideration here, Helen is that there are some basic misunderstandings of what it means to "honor" our parents. It means to respect them for their good works and good deeds and their sacrifices in raising us. But it doesn't mean to become a slave, and a "whipping post". That can become masochism. And there of course IS a reward in that masochism, or an expected reward of others (because the parent will not) praising you. It is basically a martyrdom. When the parent doesn't give us praise we seek it from others often. I think that you would benefit so much from seeking help. Often licensed Social Workers who work with life changes can guide us in setting boundaries, in working things out, and can provide us great insight into our own reasons for choosing what we choose, and in making healthy changes for as happy a life as we can manage given that the rain does fall in all our lives.
Wishing you so much luck going forward. There is such good food for thought in this thread. We often need to reach out for a disinterested and trained person to help us just sort through and organize our thought. Otherwise we tend to stay mired in the daily routine, because it is, if not "good", at least "comfortable.