The Problem With Predictions and Predictors

I do somewhat feel bad for Harold Camping. He was so certain that the world would end...again, that he convinced many people to give up everything in order to be ready for the rapture. They, of course, ended up broke and disappointed, especially when Camping backpedalled to say that he really knew all along that the "real" end would be coming in October and that he didn't give ANY financial advice to people: implying that they were foolish to sell or give away everything. But that's not my point.
My point, simply, is this: why can't anyone predict anything important with any kind of accuracy?

For example, the Mayans could supposedly predict with confidence the end of the world (shortly after Camping's date, as I understand it), but they couldn't predict the Spanish hordes that would trample them out of existence with steel and infectious diseases? THAT kind of prediction could have been useful.

The Farmer's Almanac can predict (more or less accurately) when the best time to plant crops is, so why can't prognosticators there predict the dire effects of using all that corn for sweetener instead of for food? Millions of people worldwide probably want to know the answer to that one.

Various polls can predict who will be of major importance in a political season, but nobody can predict when there will be a politician we can honestly trust to say and do the right things out of respect for the people instead of the PACs.

In short, I hold predictors in a cynical light. Nostradamus? His greatest virtue as a seer was that he was general enough so that things could be arranged ex post facto to seem as if he had called them correctly. Thus, those that wish to influence me had better refrain from making blanket statements that attempt to assure/inspire/frighten me into believing something should realize that their exhortations will be falling on deaf ears.
I predict I will ignore them.