But just because some things couldn't be predicted, it doesn't discredit the whole field. For example, maybe it could be a scientific result that the stock market is unpredictable?

It also depends on what you mean by predicting. Does economics have to be able to predict the Dow Jones in 10 years to 5 cent precision to be acknowledged as a science? There are zillions of things to predict in the context of economics.

Also, mathematics is not a science by that definition?