It is extreemly difficult and has happened I think less than 10 times in TV history, although it may be over 10 now due to Ronnie. Rotation is often said to be the most difficult game in pool, especially to run a rack. When you watched the break keep looking at the pack and how all the balls are tied up. Remember to run 147 you have to get on the black after every red, so that means you have to get perfect angle on the red 15 times in a row and of course make the black and get perfect position again. It is so easy to get the wrong side of a red and the the cb will be heading down table. Add to that the pocket and table size and its one monster task, Ronnie just makes it look like a cake walk. I guess the best indication is the prize money for a 147 break in the Worlds is usually 147,000 pounds, which is about $275,000, just for 1 frame/break!
All that being said the standard of the elite has reached such a level last year a pro player I know was making a 147 break in practice almost daily while warming up for the tournament!!!

20 years ago many of the Pros had probably never had a 147 break in their lives!