wouldn’t it be simple to tax the incremental gain or loss of capital assets

That's what happeend with bitcoins. High volitility investments are quite literally the problem with taxing unrealized gains. If something is worth $1 for 364 days, an $1B for 1 day, how much do you tax? If that day was 12/31 and it's cryptocurrency, you tax that $1B, even if it's back down to $1 and the person's total net worth is in the single digits. YES, that's an exaggeration of what's really happened, but only to exemplify. This is also a real issue with forex and non-blue-chip stocks. To a lesser extent, it's true for real estate, especially high-risk real estate investments. There's a lot of things like possibly-landlocked properties for sale where the buyer is assuming fairly heavy risk. If things go well, they got a steal. If things go badly, they have a worthless deed.

Despite the general trends of property, the value of an actual piece of property (or any investments) any time before the moment it's sold is an estimate at best. That bitcoin example above was never worth $1B if the owner was not actionably able to liquidate it for that price, regardless of the estimated worth.

I HATE to defend Bezos on anything, but attempting to tax the value of his Amazon stock is problematic because he would 100% get MUCH less than the value of his stock if he sold it. Since he's ultra-rich, I'm ok with taxing him as if he made $1B if he could only get full price if he liquidates the first $5M of $1B in stock, but that same effect will happen to people who make a lot less than him. Now, he deserves to be paying more than he does, a whole hell of a lot more, but taxes are not just anti-rich corrupt, they're COMPLICATED to get money from the rich without having unexpected outcomes for everyone.

“What? Tax the gain of a gain? What’s that?”

Not sure who you're quoting on that. Taxing the "wealth acceleration" seems bizarre and pro-ultra-rich to me. That actually WOULD punish people who make their first million while rewarding someone who makes $1B/yr consistently. I do have enough understanding of calculus to follow the ball on that one, and I don't think it lands how you think it would.