You're also not factoring in risk that comes from investing, the argument is lower taxes on investments encourages people to invest their money. If there is risk of loss, plus high taxes when there are actual gains, there is lower return on investment and therefore, less incentive to risk your money in investments.
It's already been mentioned that some of the money has been taxed once already, when the corporation pays taxes on profits before they're paid out as dividends.
I imagine large investors will just break up their investments into so many shell companies/trusts that each entity will never carry more than 1M in investments. There's always a loophole.