There is nothing special about most rich/powerful people (there are those that are genuinely exceptional of course and built a fortune because of that), they want the same things as "normal" people just on a different level.
Why does some generic reality TV celebrity or "influencer" make money from showing up at a party or turning on some Xmas lights? Surely no one really cares, there are 1000s of reality TV stars. But, it turns out some people do care, so cookie-cutter reality TV celebrity gets paid to turn up.
In "high net worth" (for want of a better term) social circles ex-Presidents are the reality TV celebrities, and the people who want them to turn up are rich, so they get paid a lot for the equivalent of turning on the Xmas lights.
I suspect its not really about the speech, it's about having them in the same room and giving your guests the chance at a face-to-face meeting.
Of course Obama may be more discerning with which events he attends, but the demand for people like him to turn up is certainly real.
