If you somehow became a billionaire, there wouldn't be enough cool things in the world to buy that would justify trying to make that much money. (But a millionaire, yes.) At that point, you pretty much have to be spending it on inventing new things or the public good (e.g. anti-malarial nets or fighting AIDS). Owning 30 houses would just be silly as you could already stay anywhere in the world and can only be in one place at one time anyway. (And why would you want that much complexity in your life anyway?)