Why hasn't someone invented a fixed breech autoloader by now to truly approach the accuracy potential that rimfire autos enjoy?
They did. It's called the Desert Eagle, and it is a gas-operated, fixed breech, magazine-fed, rotary bolt, semi-automatic handgun. It caught on just fine, but it's not cheap.
BTW, the 1911A1 configuration is entirely capable of very fine accuracy, but requires some work to be done in a very precise way. Trigger work, barrel work, frame work, and all have to be done precisely, usually by hand. Most production guns are not made to that standard, they would cost too much. Find a Colt Gold Cup and you will see how accurate a production gun can be. If you find a custom wadcutter gun, it will absolutely amaze you. One-hole groups at 50 feet is the norm, one-handed, standing.
You are talking about a custom match gun, you cannot compare it ti a beat-up military surplus gun. Not the same.