I'm not defending Bezos, just doing some napkin math to put things into perspective:

He receives $1.7 million a year in base salary and compensation from Amazon. With only $10B cash on hand, over 90% of wealth is just shares and some real estate. Lets say Amazon brings in a 10% return this year; on his $170B holding is $17B in capital gains. Humor me and say there is some new uncapitalized gains tax at 40%... that's a hefty $6.8B tax bill, 4,000x times his annual pay. How would he afford to pay that? Selling his shares or issuing new ones to raise capital would dilute share price and his stake percentage. Getting a loan using them as collateral would still leave him having to pay that back or a cycle of more and more borrowing.

While it should be a very high threshold to consider taxing unrealized gains, something needs done. They tax your comparatively minuscule $1,000s from eBay sales, jackpot win, or restaurant tips, and for simply owning a home, based off an unrealized "appraisal". Yet billions upon billions are made in stocks that appreciate in value (like a home) can't be taxed somehow?