Throwing the ball a lot doesn't mean you're good.
Luck: 627 pass attempts for 4,374 yards
RG3: 393 pass attempts for 3,200 yards
If RG3 had passed as much as Luck, he would have 5,105 yards.
I imagine I would average about a yard per pass as a quarterback in the NFL. If I threw 5,000 times, I would have soundly beaten Andrew Luck's record. I probably would have had more touchdowns and fewer interceptions than him, too...