It takes somebody that has figured out that there are 100 people (or more) that can start colts (not all are great at it) or get along with green horses for every 1 person that can teach advanced training and 'finish' a horse.
Some people just want to start colts forever and others start there (like I did starting 50 or more head a year) and get the bug and want the challenge of teaching really refined and advanced techniques. Our problem is that the people we have run into that want to finish horses want to go to work for a show trainer that is hauling cutting, reining or pleasure horses. Since we do not have a show barn and I can no longer show, there there is no appeal for those wanting to get into showing.
On the other hand, the show trainers that teach an apprentice everything over a 4 or 5 year period, usually have them leave to go out on their own and they take (or try to take) half of the show trainer's clients by under-cutting the price, when they go. I have seen that happen dozens of times. Someone starts out starting colts for a BNT, they work up to showing the 'second string' or riding the show horses at home that were not ready to take to the big shows. They get acquainted with the customers, learn the people skills as well as the training skills, and leave with half of the barn.