A big problem is that social phenomena are very complicated, just like economics in the real world. Or the weather. There are so many things that contribute to the effects we see.Steven Levitt's Freakonomics illustates how experts try to explain phenomenons with logical answers which, on the surface, appear quite plausible and are sold on to the public wholesale.
The very fact that these alleged experts scrabbled to cobble together a logical explanation shows the weakness of the foundations of their earlier predictions.