Companies have true value, so the stocks do hold real value, but the way they can be inflated skews what that worth really is. Companies have assets and revenue and profits which are measurable, but when you start looking forward things start to get murky as people try to predict future value.

Add onto that, just like people can borrow against their house, they can then borrow against these stocks, and our system is set up where they can do this tax free. So they have inflated values beyond the actual value of the stock which they can borrow against. Elon doesn't actually have a trillion dollars. If he actually tried to realize it all, he'd probably end up somewhere in the low hundreds of billions or even below that, as the market would crash as he flood it with new shares. He could try to do it over many years and he'd come out with more, but that'll take time. But he can borrow against his stock as if he had a trillion dollars which is a real problem.

I'm personally against taxing unrealized gains, but I believe at least past a certain threshold, we should figure out how to tax people borrowing against their stock indefinitely and never paying taxes. (edit: the borrowing itself also leaves a paper trail that can be used to audit and fine / jail people if they try to hide it) That's a big part of the problem were facing. They have this wealth AND they get to use it, without paying taxes. We need to tax their usage. If we tax their usage, that will alter their behaviour. They'll either keep spending and we tax it as they lose ownership, or they'll reduce spending, but keep the ownership in the companies that their shares bestow, but they can't do both.

E.g Someone worth billions will buy a house worth 10s of million of dollars, and just pledge some stock against it, and perpetually keep that loan alive until they die. Their house staff, paid the same way. They do have to pay interest on that loan, but they just can just pledge more stock against that as well.

Edit: This also works better with higher tax brackets. If they try to realize/borrow hundreds of millions of dollars, we need new tax brackets for that.