1) it is not the end of the story to say that there is no risk in earning wages through work. Lots of people have risked everything on a certain employer, company or corporation and they lost and lost big. They lost their jobs, they lost their income, they lost their benefits, and some even have lost their retirement pensions. Risk - to a whole lot more than just money - is also a major part of may occupations. Among these would be policework, firefighting, certain construction jobs, and any job where physical danger is always a factor. That is a form of risk also.
2) even if we accept your premise that investment involves risk, the next question becomes should government then subsidize that risk with preferential tax rates that are half of those for high levels of income? Why should government subsidize that risk?