Because they want to go. Mom & dad want them to go. Because trainers want to add to their fees for the show.
We live in an American culture where, in a great many sports, even beginners have a place in the competition, so they can gather participation and placing ribbons for their bedroom wall. Almost especially younger kids.
So, because this is common throughout school and independent sports, families expect it. They think they are missing out if there isn’t something for the beginner kids. Any sport. Riding, too.
Also because American riding culture often teaches steering & brakes first, proper seat second. There is kind of an Old West expectation of learning by doing. Rather than learning by progressive steps.
The European instructors have their students on the longe line for quite some time – weeks – before they are riding independently. Much American instruction (not all of it, but the more common lot) is aimed at a student who can steer and stop, make a sort-of circle, navigate cones, and ride around the ring, within a lesson or two. The rest is built later. And that is what parents want to see, because they know nothing about developing the seat. Steering and stopping look like progress, to them. Longeing looks like a waste of a lesson fee, to them.
Also many American parents with a riding background started that way. So that is how their expectations are set.