You've hit on a few reasons. Basically: taxes, no more need for money, desire to give back, desire to have a lasting legacy, societal pressure...

And no, there really is very little work done evaluating private foundations. They operate with very few guardrails and oversight is extremely minimal. It is difficult to evaluate the effectiveness of charities, so how do you evaluate foundations that give grants to charities?

I am an academic that studies foundations and would be happy to talk more about it if you have other questions.