What is it about infrequent visits?

4 out of 5 kids hardly visit mom. Too far away, too busy, both, or none. They just don't visit except once every year or so, at their convenience. This is hard on mom, but why is that? Is it because mom misses them, or is it because she mentally can't handle their sudden appearances? One kid in particular, says he'll come "in June" and then mid-May says "in August", and so on.....this is really hard for mom to get her Senior Hopes up, only to be let down repeatedly. I really want to think the best, but all I can see is the very sad look on mom's face, in her eyes, after she gets a call from them saying "hello how are you and by the way we can't make it in August.....". It is depressing. How do I help her feel good about these adult children, I don't feel too good about them (at all) but I am trying to keep the best attitude going.