Because they're too busy trying to save millions of people in third-world countries to care about first-world problems.

Less snarkily, the pitch of Elon Musk is surprisingly selfish--he's basically doing this as much so he can get to Mars as anyone can. By contrast, people like Rockefeller and Carnegie in the past are largely responsible for endowing public culture in the form of museums, concert halls, public libraries and the like, and the modern day equivalents like Gates and Buffett are attempting to eradicate malaria from the world.

Nevertheless, you should reserve ire for those who have the ability to do philanthropy and fail to engage in it in any form as opposed to those whose philanthropy isn't in the areas you'd prefer.