I really have no idea other than there's a few respected people/coaches that do hold him in high regard. So I think a lot of people/fans follow along in that belief. I forget who said it but they said Garrett was the smartest qb they've been in contact with. I know that doesn't really tell you much but what I do know is that there's a few teams that want him. Such as the Browns, Dallas, and Saban over at Alabama.
But I do see your point... none of qbs have done anything special. Joey said JG is a great coach but I haven't seen any improvement from Joey. It's just very hard to gauge any talent on our offensive staff because we have problems/excuses for everything. "Mularkey stinks" "but wait, he was forced to use Linehan's playbook" "the oline stinks, but wait, we had a rash of injuries, our receivers aren't very good, but wait... it could be the qbs throwing them the ball". And then there's our kicker... Myself, I feel like we didn't put him in the best situations to succeed. I don't expect him to make every 53 yard fg with the game on the line.
We just have so many problems that it's difficult to point out an individual as being anything special. Maybe he's the slightly better then all the other crap on the offensive side and so he stands out.