You think that stock price needs to be somehow supported by physical assets, but that isn't true and hasn't been for decades.
I buy Tesla stock because I believe that in the future I can sell it for more than I paid for it today. And I believe that because I believe that Tesla will prove that it has the right mission, and more people will want to buy into that. I believe they are doing a lot of things right, not just technologically and by betting on electric cars, but also by cutting out the dealers, which not only in America have a bad reputation.
What, you think any of the tech companies you mention have the physical assets to justify their stock prices? If you sell off all the physical assets of Google or Facebook, you can probably buy a single-digit percentage of their stocks from the profits. They have revenue streams - but Tesla has that as well. Heck, people don't even pre-order iPhones a year in advance!
There's even some questioning if overall the car market will shrink because younger generations have less interest in owning cars.
Yes, it will. I am one of those people, didn't own a car for 10 years (after selling my old one), now owning one again because I moved into the countryside. But you know what? Tesla is on the right track there as well, understanding that electric cars and autonomous driving go together. In the near future, taxis will go the way of the Dodo bird, and will be replaced by autonomous car-sharing vehicles, which you summon to your location by App and when you are done they drive off to the next customer.
Guess who is closer to having a car like that than any other company?