The basic reason for me is volume. I put 2 to 4 thousand pistol rounds through the same gun per month for action pistol, where Bullseye level of accuracy is not needed as much as reliable function.
I leave the dies in the tool head, which holds the settings to produce the round specifications I want. If I change calibers, the settings for the new cartridge are already there in the new tool head that I swap into the press.
Even with the basic Dillon auto indexing progressive that I use (SDB), I can regularly produce over 500 rounds an hour of practice ammo and 400 rounds an hour of match ammo. I'm pretty sure that I wouldn't produce that much with a single stage or a manual turret, and I'm not willing to sacrifice the volume for what is for me a small and unneeded increase in ammo accuracy. Now I might consider a Dillon 550, which is not auto indexing, but just so I can load rifle (.223 in volume) and perhaps a single stage for accuracy rifle.
Edited because, like a newb, I didn't read the whole thread and was adding to drift and tried to make my post a bit closer to the OP's question.