Honestly I think it really is as simple as what he answered in this talk: he doesn't want to risk the money. Elon on the other hand, has been willing to double

Honestly I think it really is as simple as what he answered in this talk: he doesn't want to risk the money. Elon on the other hand, has been willing to double down on bets involving 100's of millions of dollars or more. That speaks to a very different view of risk/reward between them (and not one I think is negative to either side). Also, while Carmack is certainly enormously respected in his field, he hasn't worked much outside that. Elon's track record has diverse successes, and I think that means it's a lot easier for Elon to convince investors to join his endeavors.