Why were some of the Best QBs in NFL History drafted so late?

Their talents are not recognized until later; and some are selected with over-inflated talents and become a bust in the league (Ryan Leaf - Chargers 2d Rd Pick in 1998; Akii Smith - Bengals 3rd Rd Pick in 1999; Dante Culpepper - 11th in 1st Rd by Vikings, and there are more).