There are number reasons:
1) Economics, being the dismal science, has been very bad at prediction. Sometimes, some schools get stuff right, other times they get them dead wrong.
2) Due to the huge number of variables, it's hard to even make a plan
3) With Economics, what worked last time may not in fact work this time. Economics is not like Physics. Sometimes it /seems/ deterministic, other times it doesn't. It's also very difficult to test in a "lab-like" setting. There's no such thing as a clean-room country that you can test your theories on with... (read more)