So I’m more of a show jumping history nerd in a few ways than I am an eventing history nerd - not that I don’t love eventing and its history but I’m a bit more well-versed in show jumping history (keep in mind, though, I’m in my late 20s so basically everything I’m about to write about was all waaaaaaaaaaaaay before my time.) because of a horse-related history/research project I’ve spent several years digging into. Your point about people buying made horses and how there’s such a stigma made me think.
So, look back through show jumping history - and I’m going define “made” in the absolute loosest sense here (horse is competing at a relatively high level but may have various degrees of “rideability” or definitely not be an “easy” ride).
Nautical, AKA “The Horse with the Flying Tail” (Disney movie was not at all the real story). Hugh Wiley bought Nautical from Pat Dixon in 1955. Dixon was the trainer at Millarden farm back in the '50s (read the USET Book of Riding published in the mid-70s, per Wiley’s own essay on Nautical, he needed a horse for the team, Nautical, then known as Injun Joe, was available), would probably be considered, by most standards a “made” horse, competing on the national circuit at the NHS, PA National and so on in the professional jumper classes before Wiley bought the horse to ride on the USET from '55 to 1960.
Another old-school/throwback example, in its own way - Ksar D’Esprit. Bred, actually, by Wiley’s grandfather’s farm for hunter/jumpers (there’s one picture floating around of a young Hugh Wiley riding Ksar D’Esprit circa the late 1940s), at some point Ksar D’Esprit was sold, spent time with, I think, Canada’s equestrian team w/a rider by the name of Shirley Thomas or Thompson, competing internationally, Eleonora Sears bought him and gave him to the USET. Again, probably could be considered a "made " horse.
I guess, my point here is - that’s been part of the upper levels of almost any equestrian sport for as long as the sport has existed.
My two cents, for what they’re worth? Honestly, I think people just like it better when they see the rarer instances where the rider trained the horse up themselves because it makes for a better/more satisfying/happier “story” - maybe it’s just the writer/journalist in me, but I think people just like a happy, feel-good story and who doesn’t love the concept of finding a young horse that’s maybe a bit of a diamond in the rough, bringing it up the ranks and so on. And of course, it’s great when that happens, but it doesn’t always happen!
The divide continues…
I have the same one for my dressage horse 
