That’s not the real reason, though. We often have more than twice that many horses showing at our two-ring shows, and even more at the three-ring ones. One reason is that each dressage test takes a certain average amount of time to complete. Once the person responsible for scheduling (usually the show secretary) knows how many people have entered and what test(s) they are riding, they can schedule according to the amount of time needed for each test.
That brings us to the second reason: You can’t enter a class “on the fly” like you can at a hunter show.
There is an opening date and a closing date for entries, the closing date being at least a week before the show day. This allows the show secretary to make the schedule.
Shows usually run pretty close to on time. The scratches and the tests that take a bit longer to ride tend to balance each other out.