Or let's just say hypothetically it's not. Why do they feel the need to have more users?

I'm of the opinion anyone who's still in the game after becoming a billionaire is doing so for non-monetary reasons. At this point it might not be about money, but power? Either way...

a) more users doesn't necessarily imply that (though in this case is probably does), and b) it's about the "why" - The conclusion from "more users = more money" to "must be about money" isn't universally true, as it ignores any and every other possible motivation. E.g. power, legacy, success.