• Capital gains are not inflation-indexed, which is one reason to have a lower rate. Consider three individuals:
* Person A earned $100,000 working at BigCo in 2019
* Person B sold shares in 2019 that were purchased in 2017, for a (LTCG) gain of $100,000
* Person C sold shares in 2019 that were purchased in 1965, for a (LTCG) gain of $100,000
There's essentially no inflation to account for in cases A and B, since all of the relevant transactions happened relatively recently. But what about Person C? The real value of her investment has not increased by $100,000 — it's much less than that because of inflation. So one argument for a lower capital gains rate is to be fairer to people who have held investments a long time.
• Investments are more mobile than wage earners. This is just a fact about the world: I can invest in a different country more easily than I can move to a different country, which leads to "tax competition" for investment income moreso than for wage income. However, this doesn't mean much in the US, where we tax worldwide income (so it doesn't matter where you earn investment income, for the most part).
• Capital gains is "double taxation". It is true that if you tax capital, that is likely post-tax money. That is, it was earned at some time in the past and tax was paid at that time.
There is a notable exception, however: basis step-up at death (inherited assets don't trigger capital gains when passed to heirs. If the total estate is under the current limit, I believe around $10M, then no tax would be paid at all). There are other tax preferences like the primary residence $500k exemption, qualified small business stock exclusion (look it up, startup founders!) that allow people to realize lots of gains without paying any/full tax. There are also less-sexy things like muni bonds.