So why do they not use a carrier rack in the slot?

You know, an assembly that is very perforated with cut grooves on each side that the paper would slide in (like a 4x5 combi-tank). With multiple paper grooves an 1/8" apart you could get maybe 6 prints in a carrier designed for a one inch slot.

An auto-agitator would be relatively simple to construct for something like that.

Just like makin' french fries.