First, not being motivated only by money doesn't mean he doesn't like money. More money buys more access and more funding to be able to do what he does, and the fame and attention from being one of the richest people in the world (which he had to get that from somewhere in business, he didn't start out a billionaire) lets him direct attention to causes he cares about, say 'Hey look, this is important' and people listen (at least somewhat).

If he starts a charity, it's not generating new income so he has to rely on only his own and external donations. It may be enough, with how much he is worth currently, but why not take all the free funny money from investors in the stock market while we're at it?

I'm not actually arguing that he's a charitable human being, I'm arguing he's motivated to progress humanity in a technological way (which could have charitable side effects). And either he's the head of a business making that change, or he's funding other companies to be that business, so why not have direct control over it?