Did I say mandatory? I meant optional! You’re “free” to die in a cardboard box under a freeway as a market capitalist scarecrow warning to the other ants so they keep showing up to make us more!
Realization is the establishment of value not sale for cash (it just happens that the most convenient establishment of value for any non-fungible asset is sale). There are already some realization events that don’t have associated cash flows, to do with overseas assets or certain financial instruments. Ordinary people don’t need to worry about this stuff, it’s not for them, and if you’re rich you can trivially figure out the cash flow issue.
But capital gains avoiding tax for the life of a wealthy person who lives off collateral zed borrowing, then being stepped up in basis for their heirs is just embarrassing for the US.