I can diversify my investments, I cannot diversify the company I work for. For an individual being employed is pretty risky. I'm fairly confident that I have enough money to survive a recession without needing to take a hit on my investments (and I'm assuming the uber-wealthy are definitely in this situation). I'm not confident that I won't entirely lose my main source of new income.

Another argument for low long term capital gain taxes is that the federal reserve (the Fed) can artificially manipulate the inflation rate (say from 1.5% to 4% / year) to increase for example the value of a home and cause everyone to pay higher capital gains without those tax-payers having actually gained any additional purchasing power.