> Thiel bought a lot of PayPal equity for basically zero dollars in a tax-free account.

so he did pay his taxes - he paid the amount required by the financial instrument setup by the gov't (which happens to be zero here).

And his situation is fairly unique and rare - very few people invested in a startup via their retirement account, and have massive success.

For most people, their wealth would already have been taxed; e.g., people with excess savings that they put into investments such as property or shares.