But if you put in a 40 hour week as a teacher/construction worker/etc, you are (in most cases) guaranteed to take home a paycheck. Nothing can take that away from you. Even if they fire you tomorrow, the time you've spent on the job will guarantee you get paid. Legally, you can't be denied compensation. (Ok, your company can go bankrupt, but that's still an extreme case.)
On the other hand, if you spend your time starting or expanding a business (either directly, or through the stock market) with the plan of getting rich off capital gains, you might find that at the end of the year, after all your work, you end up with nothing if the company goes bankrupt. Nada. Zip. Ziltch.
Because there is the risk of not receiving compensation, many people will require additional incentive to take that risk. That's why many countries tax capital gains at lower rate... it acknowledges both the risk, and it helps expand the economy.