1) High income and high wealth are not the same thing. People keep mixing the two together (switching between "tax high incomes" and "tax the wealthy" like they're the same thing).
2) Many rich people don't have huge piles of money lying around, but instead own valuable companies or other assets. For example, Mark Zuckerberg owns a large part of Facebook, but that's not directly convertible into its equivalent in dollars. In fact, after the IPO, he had to pay an enormous amount of taxes on that, which was money he didn't have at the time [1].
1 - http://www.forbes.com/sites/robertwood/2013/12/20/mark-zucke...