Because it conflates fit and prediction. If I take a graph of the DJIA over the 20th century then fit a polynomial to it it's intellectually dishonest to turn around and claim that I have a predictive model of the stock market. I have a fitted function, nothing more.

This is similar to the sharpshooter fallacy. If I shoot at the side of the barn then draw a bullseye wherever I hit I can pretend to be a good shot.