several things that come to mind.. One, a lot of the auto trannies come with torque converters that hold at least 12qts of oil+whats in the pan. The amount of oil alone will negate any possiblity of shearing to that point of sludging up. But in a toyota engine, the sump capacity is 5qts, and the same oil would be sheared more frequently. Some of these engines have a 7qt capacity and because of this, not one of them have been brought in for sludge. Same motor, same pcv, and same everything except engien is mounted in a wider open area, allowing for better air flow around the engine, and the higher capacity of oil.
Another thing on trannies, There is no induced blowby unlike an engine produces, so you don't have as much acid build up trying to oxidize the oil.
I'm also trying to remember but I don't think there is all that much in the way of gears in an automatic. Theres the valve body, which shifts the fluid around creating the shifting, then theres the clutch pak, that doesn't effect it as a gear would, the torque converter has vanes in it, although there is an oil pump which would be simular to an engine oil pump.. So, question I would wonder about, is the amount of gearing in an automatic trannie = to or less than a normal engine? I don't think you have near the gear area in an automatic thus less shearing of the oil and then factor in the quanity of oil used - the acids from blowby.