Capital gains is taxed well below the normal tax rate so it really does not add up to as much as you might think. "As a share of total federal revenue from all sources, capital gains taxes averaged about 4.2 percent from 1995 through 2009." https://pocketsense.com/percent-irs-revenue-comes-capital-ga...

Further, it's skipped when someone inherits money making death a massive tax break.

PS: 34% of federal revenue comes from payroll taxes which are very regressive, 47% from income taxes which include capital gains which are progressive. Net result a surprisingly flat effective tax rate.