Why don't professional bowlers score higher?

It appears the average professional bowler scores around 230-240 out of 300. In my opinion, this should be much higher.

In bowling, unlike other sports, there is one movement you need to perfect: the movement that gets you a strike. There are no other players to tackle you; just you, the ball, and stationary pins. The fact that pros rarely get a perfect 300 leads me to believe that there must be significant variables that I am missing. All I can think of is the ball being used (size, weight, etc.), floor friction, and pin size and weight. However, this doesn't seem enough to account for this semi-low average as these variables will only ever differ by a minuscule amount.

Why is this average so low? What factors contribute to this "semi-low" average?