You tax it when they take the money out to spend it on themselves
As long as creating a legal fiction is a mere matter of having someone else do paperwork, you either need to extract tax from legal fictions, or kiss a big chunk of taxable activity goodbye.
The two main tax avoidance strategies would be people running and retiring overseas just before selling, or never selling and waiting for the cost basis to be reset upon their death.
Spending would turn into various non-monetary and indirect compensations. It is going already but would get even more efficient.