Actually, it is a good and a fair question. The simple answer is, they don't play enough to be at the top level. No mater how much knowledge you have, you still have to put in time on the table. When you know WHAT to do, you can put in a lot less time than someone that doesn't know what to do and has to figure it out by trial and error, but you still have to put in the time.
Most instructors have figured out that you can make a lot more money teaching the game than you can playing the game. There is no money in playing the game except for a very few. And, those few will change from year to year. So, looking at it long term, forget it financially.
I don't care who you are, you are not going to play your best unless you are out there doing it all the time. Those pros that don't play all the time still have to practice for a period of time before they go getting in a match. The old proverb of "use it or lose it" fits very well with playing your best. Your knowledge will stay. Your finesse will suffer with a lay off.
So, you end up with guys like Jerry Briesath. Guy can't even get on Joey A.'s list, yet he is one of the top instructors in the world, and used to be a pro player. He doesn't play professionally anymore, he instead has taken his pool career to a different aspect, namely, teaching. He isn't about to keep up with the current pros in playing today. Why? Just because he doesn't play anymore.