What you makes you think Tesla is just an engineering problem?
You have regulation, laws protecting dealers, government negotiations, lobbying (and that's just from top of my head) and not to forget, large distributed software and hardware infrastructure
SpaceX: strong relationships with customers, dealing with lots of government and private companies that are paying you millions of dollars, etc.
> You're assuming that the attributes that lead to success in those companies will also carry across.
There are many attributes across the board that is needed to succeed in a scale of those companies. Even in Musk's existing companies, he's most definitly not suited to do certain tasks. It's well known and that Gwen Shotwell is a huge part of SpaceX'es success to date. I agree that social network requires its own area of knowledge and experience, but Musk isn't literally doing all this by himself. He attracts talent and that's the key.