Life would be a lot easier if what you're saying were true but unfortunately, things are a bit more complicated.
Maybe hundreds of years from now, we'll be advanced enough as a species to know what the best solution to pretty much any economics-related problem or at least advanced enough to have a foundation that's as solid as what we have with math for example but right now, that's not the case (not by a long shot).
Therefore, it's not an exact science... at least, not yet.
Let's analyze the math analogy:
We know that 1 + 1 = 2, there aren't any debates as to whether or not 1 + 1 = 2 and to extrapolate, there aren't any debates when it comes to the "foundation" of math.
The same thing can't be said about economics.
We have the current financial crisis as a more than eloquent example.
Keynesians think that central banks should print money... to them, this solution is "textbook economics" as Krugman often says.
Austrians think central bank involvement via money printing is the worst possible solution, that's "textbook economics" as far as they're concerned.
IMO, we can't say economics = exact science until there can at least be consensus as to what exactly constitutes "textbook economics"
Maybe it will be an exact science in the future, maybe not.
Right now though, it's a combination between an exact science and an art form, the art of capital allocation or whatever people choose to call it.