I believe that many semi-autos are capable of very good accuracy, mechanically speaking, (fired from a Ransom Rest). For most shooters, I believe it comes down to good sights and good triggers. The longer the sight radius, the more precise you can be. 3 dot sights are probably the most precise for most people. Fiber optic front sight also help.
As for triggers, if you took the most accurate gun in the world and slapped a heavy, gritty, and creepy trigger on it, no one would shoot it well. I light, clean, crisp trigger allows you to fire the gun without moving the gun.
I'm pretty sure the guns you listed as great shooters had great sights and great triggers on them. After great sights and triggers comes the gun's tolerances. This is where the cream rises to the top. A lot of the higher end 1911s (Wilson Combat, Les Baer, Ed Brown, Nighthawk, etc.) build their guns to very tight tolerances and use match grade barrels and triggers.
I have one of Les Baer's 1911s with a 1.5" guarantee at 50 yards. I'm not saying I can shoot it that well, but the gun is capable of it. Most people who have shot my Les Baer shoot it very well, and that includes total novices. My Sig P210 and STI Edge are also capable of incredible accuracy.