I don't believe 20% of amateur boxers turn pro. That is ridiculous. Their definition of amateur must be like us saying 20% of Short-stops turn pro, instead of us saying 20% of D, C, B, and A players turn pro.
As far as skill goes, I think in any sport, the difficulty of reaching the top is the same, from a purely athletic ability. No matter how easy or hard the sport is perceived to be, it will always be a bell shaped curve of where people end up in their abilities in said sport. You are not competing against the sport, you are competing against your peers. Not everyone can be at the top, it wouldn't be the top then! LOL
Now, as far as the business and political side of reaching the professional arena of sports, pool must be one of the easiest on the whole planet. All you have to do is put your money up to play in professional events. And if you have the skills, and do well in those professional events, you will automatically be called a pro by your peers. In other sports, you have to jump through hoops just to get a chance to play on the same arena as other legitimate top pros, even if you do have the skills.