Pin-nut is right, until you've bowled on the patterns, you just don't know how difficult they can be.  However, what 01kay said is somewhat correct, the more you bowl on the PBA patterns, the more experience you get at understand them and what you need to do to be successful.  The biggest difference between the successful pro's (WRW, Duke, etc) and the rest of us is not so much the physical game as it is experiece at that level of competition and experience with the PBA patterns, environment, etc.  The more you compete in that environment, the more you learn and improve.

My local house rotates the standard patterns for a summer league, so I get to practice on them quite a bit. To make things very general, the thing that makes these patterns so difficult is that it you do not have the ability to create area like you do on a THS. As was already stated, miss inside and you're going through the beak, miss outside and you're either washing out or taking less than a 5-count. The PBA patterns require consistency in terms of accuracy, speed, angle, revs, etc on each and every shot. You can't just take a spare for granted; you must focus on hitting your mark with a solid release on every single shot. The guys that we see winning the money every week are the guys who have mastered the ability to do that. To them, it probably is almost second nature at this point. That doesn't mean that it's easy, that just means that they know what they need to do and they've mastered the ability to do it.
--------------------
Bowling is without a doubt the dumbest, most pointless, most idiotic excuse for a game that has ever been invented. So, what time are we bowling tomorrow?