Why aren't the trades on stocks taxed?

It came to my attention recently that the majority of Wall Street investing is actually gambling. This trading does nothing to help develop needed innovations or businesses so desperately needed for us to meet the challenges of our day. It seems it would be a very good idea to put a very tiny tax on each trade. Such a tax would hardly be noticed by the occasional or serious investor and could lead traders to invest in the countries future. If the traders are not deterred by the tiny tax then quite a bit of revenue could be raised to help pay on the national debt or used for the general welfare of our citizens. I understand this tax was in existence before. Could it help again?