There is incredible volatility to playing quarterback in the NFL. One play you're standing in a clean pocket, with perfect view of a receiver route tree, while stepping into a pass that will certainly result in a touchdown. The next play, you're can get blindsided, or hit in the mouth, or fumble the ball away because of a missed assignment by a teammate, or making a bad read and throwing an interception.
No position in sports has that kind of diverse outcome from minute to minute. You're a hero or a goat on almost every play, with very little grey area.