Why do grown children ignore their parents?

It depends on whose eyes are regarding the situation. To an outsider, it may appear as you describe. But to the grown child who actually knows the situation, going low contact or no contact may be a necessary action to protect oneself from a toxic on-going relationship.

Parents didn't use to live until their 90's and 100's.

Often the parents are a nuisance.

A variety of reasons. Probably the most common one is that you did a good job raising independent hard working children that have responsibilities.

Another question to ask might be "Why do parents drive their grown children away?"

It's the natural order of things, I think. Children grow up, start their own lives, create their own families, focus their energy on the next generation, and the next. Is there any species in which the grown children stay behind to devote themselves to the generation before them? I don't know of any.

Raising children is hard. Each generation needs to be able to focus on this difficult task, or the species would die out! I think that is part of the evolutionary process. Each generation is responsible for the next. Humans have a long period of dependence!

That seldom means (in humans) that there is no contact or warm feelings or continued caring both ways. Certainly not all children consider their parents a nuisance. During their own child-rearing years (a couple of decades or more if they have several children) they may consider attention to a parent more than they have energy for.

Look at the many, many members here who are caring for an elderly parent with love, with patience (more or less), and with sacrifice. They may be frustrated but they aren't ignoring their parent.

And there are also plenty of individual cases where the relationship is toxic.

Sometimes you reap what you sew. My parents once hopped in the car at night and drove an hour to support my sis when her firstborn had the croup and she was worried and stressed. On the other hand her inlaws often visited another relative who lives on the same street and never came over to see their grandkids.

My hubs finally had "words " with his Dad... if you keep one kid in the dark don't expect him to provide all the light! Nuff said!

My therapist recommended (or rather strongly advised) to not see my mother.
Ten years later, after not speaking to her (and she hardly noticed), she came to me for a place to stay.