Just to throw in my two cents here - I actually work at a bowling alley/pool hall in Chicago. We have 16 9ft. tables and 16 BPA regulation lanes. We get maybe twice as many bowlers as we do pool players (our lanes have waiting lists quite often, where our pool tables are always open), but this may be anecdotal.
Have you ever bowled on a tour shot. To compete on tour you have to be able to hit a spot on the lane the width of a piece of black tape about 45 or 50 feet down lane with your bowling ball. And if you miss you might squirt out the back or go through the nose. I believe to be a pro in any sport it is difficult.
However I will say this. At this point in time I would much rather play pool then bowl.
I agree that being a pro in anything is insanely difficult, but I will also say this - both games involve the pursuit of a recognizable perfection (in bowling, a 300, in pool, not letting your opponent get a chance to shoot). However - in bowling there are WAY less variables involved. Lane conditions are pretty predictable after one frame, and after that, you're throwing a ball towards the same position every time. In billiards, there are WAY more variables - position, table speed, stroke replication, banking, kicking, safeties, combination shots, and that's all just in 9 and 8ball alone.
Also, when I started working at the bowling alley/pool hall I had never really bowled before, but I had been playing pool for 2 years. 3 years later, I average a 175 in bowling, but still have trouble stringing more than 2 racks together in straight pool.