That is an excellent question. If I could use altitude training I would. Simply put, it is not a practical option. In Europe and other racing jurisdictions, it may be the case that a trainer has the ability to train thoroughbreds at a certain height above sea level with which to employ altitude training. They would be in an enviable position! In North America a racehorse must be somewhat close to the racetrack if not training from the backstretch for simple reasons: starting gate approval, published workouts, access to a consistent and safe surface to train on, access to suppliers (feed, hay, farrier, veterinarians, good riders, good grooms, etc). So, at the margin, for an increase of red blood cells, you may possibly be giving up the ability to have access to the most highly qualified professionals which are crucial to the racehorses' success.
I hope this may answer your question, I agree it is a wonderfully useful method of training.