>Why aren’t capital gains taxed at a higher rate than income?

The federal capital gains rates are higher than the effective tax rates paid by a family making a median income, but I suspect you are asking why the capital gains rates are not higher than the highest marginal rates.

One issue is simply that capital gains tax rates generally don't account for inflation. If you build a business over a few decades and sell it, much of the increase in value will be simply due to inflation. Do you want to encourage long term investment, or make it so only financially illiterate people do long term investments?