QB stats tell you what happened, but they never tell you why they happened or who performed well or bad. There's not a single QB stat that is the sole result of the QB's performance, so it's impossible to truly evaluate a QB's performance without watching their play on tape.
And the NFL professionals analyze every play of a player, they don't base their evaluations on just 1 or 2 games, or some highlight reels. They study the All 22 version of every play.