Because they stopped thinking about the future. Very very few people are willing to plant a tree whose shade they will never lie in. In other words do not willing to give up much of anything even trivial things to make the world a better place.
As an old person your head gets stuffed full of bad ideas especially between the ages of 4 to 14.
Young people at least have the excuse that they're young and dumb but old people had decades to deconstruct and get the fuck over those bad ideas and they just don't. Instead they double down.
Old people are worse because they have hindsight. They're also much much worse here in America at this moment in time because old people, basically anyone over 50, got the full benefits of the new deal and great society and they yanked that ladder up behind them. Again for trivial reasons.
Basically what I see with old people is you've got somebody to spend 50 or more years on this planet and didn't learn a God damn thing. That's what makes them worse. I can forgive an 18 year old for doing something stupid. Not so much for a 50-year-old.