I got an education that gives me a salary so I could afford to buy both my horses. I ride hunters so while they are still expensive, they aren’t grand prize (FEI) expensive.
A lot of those riders have sponsors or syndicates to buy horses that can jump at the highest levels of competition. They also have sponsors and networks that help
pay for the expenses or have family money that helps. Keeping and showing those horses is VERY expensive over and above the purchase price of the horse.
They create their reputation and rider rich owners horses. (sorry for generalisation!)
A lot of the top riders don’t own their own horses, when you are a good enough show jumper, rich people who own horses will get you to ride them on the Circuit so they are worth more
My parents gave me my horse, she jumps Grand Prix, however, it was an expensive investment they did with me and I have to show results or else they will stop investing.