其中一个原因是避免双重上税（投资本金已是税后的了），见：

the capital that you used to generate a return (that is taxed by way of cap gains) was already taxed simply by way of using those income dollars to purchase financial assets.

https://www.reddit.com/r/AskEconomics/comments/13pwcy8/why_are_income_tax_rates_so_much_higher_than/#