Some don't donate due to gratitude, some donate to increase the chances their children will get in, and/or otherwise obtain social signals or networking benefits that can be used to further oneself.
We had a large percentage of my graduating high school class end up at ivy league schools. It was interesting to talk to them a decade after they had graduated and were in full swing with their professional career's and life.
One buddy I know who went to Princeton and ended up on Wall St told me they hound him for donations. His fraternity does the same with the emphasis on exactly what you pointed out. He wants his children to have a better chance they he did, so he donates large six figure sums every year to the school and fraternity hoping when his two kids grown up, they will have a leg up on the competition to get in there.
As a fellow Princeton grad, I think his children will indeed have a better chance than most of getting in due to their legacy status, resource rich upbringing, and those donations.
