I bet there is maintenance, a horse running 5* probably needs more maintenance to stay happy than just be-bopping around at home?
Plus trailering, probably XC schooling fees, extra lessons to be competive, costs to trailer out to condition on hills or water treadmills?
I’m guessing whoever is not competing the 5* horse is because the funds aren’t there to be competive, not to not enter? The professional is making more money spending their time riding young horses for owners who pay all the bills then spending their own money to run 5*.