Because if he's able to help anyone on SO, he very likely profited many, many times from free access to knowledge there before he got to this point. Given activity of average user he probably gained orders of magnitude more than he given.

I find rambling about money and compensation in such context distasteful.

SO provided platform which, while not perfect is used by millions of people. They aren't overloaded with ads and dark patterns as many of the clones. If it's worthless, why people are using it instead self hosted blogs for example?