Why is mother with dementia unable to adapt to my moving in with her when she so clearly needs help with every aspect of her care?

If she has anosognosia and dementia, the disease has affected the areas of her brain that allow for adaptation. Permission from a person with non-existent short term memory means...she's forgotten she gave you permission. And she won't remember no matter how you try to explain it. Your being in her home is a disruption to her routine, and when there's no working short term memory, routine is the glue that allows a person with dementia to get through a day. It may be a complete suprise to her every morning to find out that you're living in her home!
Her grasp of why you're both living in her home may be foggy, and in conjunction with her anosognosia may mean that anything you do to try to manage her day or point out her losses is going to seem like you're inexplicably trying to manage her life, and she won't like that. And so she's angry and confused, and lashing out, because the disease ensures she can't retain information, manage her emotions, identify that there are issues, or see that she needs help.

Read the article lealonnie1 has recommended. Also watch some Teepa Snow videos on Youtube.
I like these, too:
5 losses: https://www.youtube.com/watch?v=awBm4S9NwJ0

Careblazers-5 mistakes to avoid: https://www.youtube.com/watch?v=rO50pVUOlbE

Talking to a parent: https://www.youtube.com/watch?v=oiUpztj2DkQ

Stepping Into Dementia’s Reality: Advice From Teepa Snow | Brain Talks | Being Patient:  https://www.youtube.com/watch?v=EOCZInnLQd0

Re read the first 5 words of your question. The answer is right there.
She has dementia.
People with dementia have a difficult time with change.
Even more so if she does not think she needs help.
You have made a HUGE change.
You and your husband have moved in to her home.
In her mind you are the ones that do not belong.
She may adjust. If she does it may take months. Or she may never adjust to having you there.

I suggest you read this 33 page booklet online about managing dementia and what to expect with an elder who's been diagnosed with it. Lots of Do's and Don't tips for dealing with dementia sufferers are suggested in the booklet.

Understanding the Dementia Experience, by Jennifer Ghent-Fuller 

https://www.smashwords.com/books/view/210580

Jennifer is a nurse who worked for many years as an educator and counsellor for people with dementia and their families, as well as others in caring roles. She addresses the emotional and grief issues in the contexts in which they arise for families living with dementia.

The reviews for her books are phenomenal b/c they are written in plain English & very easy to read/understand. Her writings have been VERY helpful for me.

The full copy of her book is available here:

https://www.amazon.com/Thoughtful-Dementia-Care-Understanding-Experience/dp/B09WN439CC/ref=sr_1_2?crid=2E7WWE9X5UFXR&keywords=jennifer+ghent+fuller+books&qid=1657468364&sprefix=jennifer+ghent%2Caps%2C631&sr=8-2

Educate yourself about what lies ahead as dementia progresses before you decide what steps to take. Knowledge is power.

Good luck.

Adapting is not something that folks with dementia do well, if at all.

I think you need to do some reading here and re-consider this plan.

Your mom may "adapt" far better to being in an entirely new environment like AL or MC.

From your profile: "Husband and I moved in with mother with dementia 4 months ago. It has been very challenging to say the least. Very little help from siblings who live locally. My husband has not wrapped his head around the fact that my mother is not "making" this up and behaving abhorrently on purpose. I am always seeking support and ways to support my mother."

Why were you the one to sacrifice your living situation to move in with her? Did your sibs perhaps want her in a facility, and so aren't willing to help you keep her at home? What kind(s) of help were you expecting from them?

So you've been living in this negative environment for four months now. Sounds like your H might be having regrets about this plan.

"She has anosgnosia".
You've answered your question.

You've also made massive changes to your own lives, including moving into a basement, so that your Mom doesn’t have to move or change.

Was this just a temporary crises solution? Or is this the long-term plan for all of you? Was this a decision your Husband is ok with?

All the problems are due to her dementia. Has she been tested for dementia? Did they run blood work to rule out certain things? MRI? I'm not saying she doesn't have it, just saying there could be some issues. that could be addressed to help her a bit.

Change the story about why you are there to something that might not annoy her. If you're telling her you're there because she can't do X, Y and Z anymore, that could be setting her off. Instead, join the therapeutic fibbing club. Tell her whatever works for you and might calm her down. "Thanks for letting us move in. This is so much easier on our finances. And it's nice to be here to help you if you need help."

Is being aggressive and cruel new behavior for her? I hope so and if so maybe she could benefit from a small dose of something for anxiety? IDK but her behavior it no the best and everyone could be happier if she weren't being so difficult.

Dementia robs people of their abilities to use reason and logic, so they can't form good judgment any longer. They lose their ability to empathize with others, they lose the ability to be centered in time and space (and memory loss exacerbates this), Not only can they *not* retain or remain at stasis, they are actively losing abilities every week. They are not able to readily adapt to anything anymore, no matter how good it is for them.

I found Teepa Snow videos on YouTube to be very helpful. She's an expect in caregiving for people with dementia and memory loss. You and your husband will now need to adapt to her, and learn strategies to interact with her so that you can co-exist with her more peacefully and productively.

You mentioned she is rapidly declining, please take her to the Urgent Care or ER to be tested for a UTI, which are extremely common in elderly women and can amplify her dementia symptoms. If not treated, can become septic and life threatening. People with dementia often cannot recognize the pain or symptoms of a UTI like a younger person, so even if she isn't complaining, it would be a good idea to discount this.

Also, I'm hoping one of you is her DPoA. If not, you may be in for some stressful struggles if you need to manage her affairs legally and she doesn't cooperate.

The answer is that she has dementia and her brain is not functioning well.