Elon Musk didn't start out with billions of dollars, and he has almost no liquid assets (he borrows against his stock when he needs cash).

But this is a good point. Lots of talented people have good ideas that end up going nowhere because of lack of capital for implementation.

He spent all his money on his trio of companies by 2008. He could've relatively speaking for people worth a hundred million to billions, be worth very little today.

I don't think he has ever in his life had a billion in liquid assets. You could argue I guess, but he likely wants to keep a majority stake in SpaceX, and likely doesn't want to go below 20% stake in Tesla.