Because while his college tape might show flaws in his mechanics, the rest of it is a sizzle reel of him making mince meat out of the best programs in the highest profile contests imaginable.
And while he might lack in some skills, others--size, speed, athleticism, arm strength--are as good or better than virtually every other NFL QB. I'm not saying he's without flaw, but you can't just gloss over the part where he has also elite level skills that don't come around too often.