I feel like this is a bit of halo effect thinking. At some point, you have to specialize. No international show jumper is also Eventing and doing Dressage and any other discipline at the same level. I have known US riders to go to smaller jumper or dressage shows as practice but likely they are going to bring that horse to an event since it is easier to just do one or two phases with that horse, and whatever with the rest of the horses - all at one place instead of trucking one horse to another show.
Also, the US has Western riding, which is something Europe and most of the rest of the world does not have. They are very different ways of going and it would be very difficult for a top Western horse to do well in the English world and visa versa. The US breeds DO have all around champions - Appaloosa, QH, Paint - these horses go English and Western to win All Around.
Most people I know that focus on one discipline “play around” with others, whether it’s jumping their dressage horse once in a while or going to a local show outside their discipline. I do live in an area that is more similar to Europe in proximity to show venues big and small. If you are out west, people really have to decide it they want to trailer their horse 5 hours (or more…much more) to one show or another and will obviously choose the one that is in their discipline focus so you are less likely to see them out elsewhere.
There is a lot more cross discipline interest in the US than I think people realize.
I also don’t think other systems are more merit based so much as they have more government money but the people that can pay the initial fee to get infront of the government (through going to the big shows) are still going to be the ones getting the money. The government isn’t out in the boondocks pulling some diamond in the rough out of the backwoods of Scotland to ride for the UK.