[greenpizza13]

Computers trade most effectively with technical indicators. These are things like price history, volume, etc. These trades are very effective in the short term, where trades can happen in terms of seconds or milliseconds. This is where computers excel and humans fall short. This sort of trading is based mostly on breadth. If you look up the fundamental law of active portfolio management, you'll see that breadth is less effective (exponentially less so) than skill is.

In the case of the long term, however, trading is done with fundamental indicators. These things can be more or less intangible and have to do with market events, people, and other indicators of company value that are hard to translate into math. Using fundamental indicators for portfolio management is what humans are better at, and these pay off in the long term (see Warren Buffet). These trades are done with skill, which, as I stated earlier, is exponentially more effective at creating gains than breadth.

In short, it takes a huge amount of breath to get the gains required by a relatively small amount of skill. Computers are better by far at breadth, while humans are better (for now) at skill. This, I think, is why humans still trade.

[wbsgrepit]

Not to mention the spectrum between intuition/innuendo and flat out insider trading knowledge that is cornerstone to the market -- trading algorithms don't do well against this human factor.