Well, you aren’t paying taxes on it because you haven’t profited off the increase in value yet. The profit occurs when you sell, hence the taxes.

Like if you own a baseball with Babe Ruth’s signature it will go up in value with time, but you won’t actually get any cash from that unless you sell the baseball. Let’s say you were taxed on that increase in value before selling… ok, so you pay the tax even though you haven’t actually gained any money yet. After you pay the tax, you have the ball re-appraised and find out to your surprise that the signature is fake. Suddenly, your ball is worthless and you just paid taxes for money that you never actually had. Stocks work on a similar principle because “value” isn’t the same as “profit”