Not that he’s flawless, and not to fangirl too hard here, but there is a serious lack of insight demonstrated in some of these comments.
I can name several reasons.
How many models of car are exactly the same dimensions? Having a precise (to some degree) fit to the tunnel is undoubtedly required for the design, or else, those guide wheels on the sides wouldn’t be in use. If you have a single oversized cart/skate/whatever, then the vehicle rests on top as that device handles the rest.
How many cars can reach 150mph? Not that it’s impossible to get around, but my truck has a governor at 100mph that kicks the speed back down to 90mph once reached, before reengaging the transmission. If the skate is designed to push the heaviest of civilian vehicles to 150mph, then all vehicles are able to reach the same velocity and you suffer less/no congestion.
How many vehicles have smart driving technology that functions equally in all situations? One of the biggest hurdles such an idea will face is usage prior to the adoption of the required technology by all makes and models. If the skates are all uniform in their activity, you don’t have to worry about people trying to take “dumb” vehicles down, or a different company’s faulty AI crashing in your tunnels.
I’m sure there are more, but you should get some idea.
As an American living in Japan right now, I can testify that trains can be wonderful things. They can also be incredibly irritating. They never seem to stop quite as close to my destination as I’d like, and there’s no way for me to bring my vehicle along with me (and there are plenty of reasons to want to bring your vehicle with you, including but not limited to simply passing through the city with no intent of stopping).
I read an article some time ago that was interviewing an engineer that used to work at SpaceX. You are far, FAR underestimating Musk’s intelligence if you think he’s just a playboy throwing money around: he often has incredibly specialized knowledge on many/most aspects of the projects he works on, and asks questions of his engineers that the interviewee claimed to be completely unexpected of someone outside the field.