It is true that they often choose mark securities to market on their balance sheets, and by doing that, they need to put that change in the balance sheet on the

It is true that they often choose mark securities to market on their balance sheets, and by doing that, they need to put that change in the balance sheet on their income statement. When that happens, in accrual accounting terms, it's income. Those markings don't create cash, though. In cash terms, they are no different. Individuals are largely taxed in cash terms.