[jackgavigan]

It's impossible to create an algorithm that encompasses all the factors that contribute to a company's stock price (or, perhaps more accurately, all the factors that a given investor believes will affect the stock price). There's also an implementation gap between the model a stock analyst can create in Excel and its implementation as an algorithm. The former is within the reach of far more people than the latter.

Finally, don't forget that somebody has to design and write the algorithms.