Same reason they hated him while he was at Auburn, and were 100% certain he would fail in the NFL do to "poor mechanics."
He's one of the most dominate players whenever he steps out on the field, and he's not even that good yet, he may never reach his best, and that's insane when you think about it.
Cam isn't a technician/anticipation thrower, he's a sight thrower. He's more Favre/Cutler than Peyton/Pennington. It shows in his accuracy, not just completion percentage overall but his completion percentage based on distance and area of the field.
If he can evolve his game a bit more, with his physical talent to make sight throws, that'd be terrifying.