Adding to the excellent breakdown provided by u/JustHereForCookies17 - in addition to parents seemingly being naive and not doing necessary research, in general there's a lot of hubris in young, inexperienced riders. They are teenagers after all, it's part and parcel of being a teenager to think they are immortal. :) If they are anything like I was (I was a pretty stereotypical horse girl), they'll think that they will be able to form a special connection with the horse (see any oodles upon oodles of pony books; Black Beauty etc) that no one else has and thus control a horse far beyond their actual ability. Same kind of thing as when people get dogs that are far bigger, aggressive or smarter than they actually can handle. The part of wanting a status symbol that is known to be "difficult" to show off that no, they (the owners) are special indeed and can do this. Usually the hubris gets toned down fast once the rider gets their clock cleaned out by 400kg+ animal that behaves like a real animal, not a My Little Pony.