You're right, it's actually 0% with any amount invested, $100,000 or $1,000,000,000,000,000, doesn't matter, provided you don't sell more than roughly $40k, or, god forbid, generate additional income through, say, work.

That flat tax rate certainly scales nicely the wealthier you happen to be, paying a max of 20% if you hold for more than a year and decide to cash in on this now several year massive run up in the markets.

It was most likely already taxed.

If I invest my paycheck in the market, that money was already taxed at 30-40%. Now I make a dollar in the stock market , and it's taxed again at 20%.

This get ridiculous fast.