Why don't pro bowlers score higher?

After a bit of research it appears the average pro bowl is around 230-240 out of 300, but in my head 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, no other players to tackler you just you the ball and stationary pins. The fact that even pros rarley get a perfect 300 leads me to believe that there must be significant other 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, but this doesn't seem enough to account for this semi low average as these variables will only ever differ a miniscule amount.

So why is this average so low? What factors contribute to this "semi low" average?