You can’t, under current US law, tax something that hasn’t been realized. Say Musk when he owned his McLaren F1. He paid, in round numbers, $1M for it. Well now they trade at around $20M. We don’t say “you only paid sales tax on a $1M car, now it’s worth $20M you owe us more taxes.” When he sells it (he already sold it but bear with me), we can hit him with capital gains tax, but until it’s sold, it doesn’t matter, he hasn’t realized that wealth.
It’s the same way I can avoid taxes by dumping $18k or $19k or whatever it is per year into a 401k. I’ll get taxed on that money when I realize it (dump it into my bank account). Until then I don’t have access to it, so I don’t get taxed on it.