One of the main reasons (perhaps the only reason): humans are complex and complexes of humans are…well…complex.
Also, here’s a great line for all the stock pickers out there:
“We’re almost always wrong. Our reputation for being right relies on some human capacity to marvel at the times when, yay, you got it right! Arthur Clarke predicted communications satellites and things like that. Those are marvelous — it’s great when someone gets it right, but almost always it’s wrong.”
He’s talking about writers with that first “We” but he could equally well be talking about anybody that seeks to predict the future, notably, stock pickers.