Because, if it’s ageism as in criticism of older people, which it generally is, they haven’t grasped that, one day, they will be the older people they complain of now.
They will be the ones slowing down, because that’s just what happens. They will be the ones whose lives are smaller, because that’s what happens. They will be the ones having age related illnesses, because that’s what happens.
When you’re young or middle aged, busy working and raising a family, you can’t comprehend that there will be a time when you’ll be the one being slow in the supermarket, slow walking along the pavement, slow paying for things. But it will happen and those entitled arseholes who are so critical will be in the same position as those they castigate now.
I don’t know of anyone who is keen to screw over young people. I know of older people who have supported the younger members of their family in many different ways and I know of middle aged and young people who are critical of the youth that lacks resilience and believes themselves to be entitled to special treatment.