This treatment of secondary currencies (regular ones included) is not universal across the world.

Because Bitcoin is property just like stock.

That's my point; it's treated as property, not currency, which sucks.

> what good is that if I have to pay capital gains tax

You would (almost) never pay cap gains on cash even if the law required it, because cash goes down in value over time. Everyone knows this, so no one holds cash, they buy stocks, bonds and realestate. When it comes time for the Tesla, they have to sell those assets, pay cap gains, then buy it, just like BTC. If you wanted to avoid cap gains, your only choice is to hold cash, in which case you would have less overall money at the end.

tl;dr: all appreciating assets are encumbered with cap gains, the only way to avoid it is to buy non-appreciating assets, which is worse