No, it's not because they are better off. It's because (a) to get control of that level of economic resources in this economy, you generally have to do something with many questionable aspects to it, despite its (potential) popularity and (b) you don't need to remain a billionaire or show signs of trying to increase your holdings. Mackenzie Bezos shows the way here - still deliriously rich but giving away money like a bank on fire and having plain her goal to ultimately donate essentially all of it.
The billionaires we don't want at FOSDEM are not just rich, they want to be richer and they are, from our perspective, demonstrably causing harm.
Yes, I happen to think that marginal tax rates above some threshold should probably be on the order of 95%, but I don't begrudge people making a lot of money by doing something good and/or for a lot of people.