The high average in my "classic" league last year was 238. The guy had to shoot a 7 each week just to maintain his average.

He's very consistent, looks exactly the same from shot to shot, and he's athletic. He has great timing and balance, and probably has very good eye/hand coordination as well.
But the reason he averaged 238, rather than 215-220, is because it's the same easy shot every week, and as bamaster says, you're only bowling three games, so the shot doesn't break down. If you can master whatever little trick that's required to score high at that house on that shot, you can strike for days.
To be a professional bowler, you have to be versatile, and you have to have a bag full of tricks, not one or two. You have one or two balls to get lined up on a pair, not half a game. You see different patterns, laid down on different surfaces, every week, and you have to be able to average 200+ on all of them.
The house star may have the physical tools necessary to bowl professionally, but it's a learning process that most people are not willing to go through. Until you bowl against the pros, on their shots and under their conditions, you're still just a house mouse, even if you throw 700+ every night of the week.
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seadrive
Cogito ergo bowl