Capital gains in retirement accounts are tax-sheltered (in a Roth IRA, completely tax-exempt). And it's a serious exaggeration to say that "most" Americans are going to use capital-gains for retirement. For the vast majority of Americans, capital gains are a negligible proportion of their total income. Raising capital gains rates and using the proceeds to lower regular income tax rates (to bring the two tax rates closer together) would lower many more people's taxes than the reverse would.