It's not about altruism or lack of it. Nor ultimately even saving money in taxes. It's about having enough, so there's no point in owning more than you need.

He obviously has to live somewhere and eat, so it's only pragmatic to arrange that he will get what he needs but, I assume, not really much more.

That was my first thought as well, but in reality, there's more available for whatever charity eventually gets the money. Granted, he could have donated it all outright, but there's something to be said for having some income so you can work on things you love, including giving back. I'm guessing that for someone like Derek, his time and expertise is more valuable than his money :)