the main reason for “incredibly hard cushions in the exit row” is the fact that the *entire* emergency exit opening has to be unobstructed for evacuation

depending on the actual interior configuration, the cushion on the window seat in particular may have to be ~1/2” thinner in order to meet this requirement, and the lack of padding makes the seat feel a lot harder

full disclosure — I’m only speaking from experience on America West A320s ~1995 (I know I’ve been in exit rows more recently — Delta 738/739/757/753 and A319/320, and AS 73G/738/739), but I can’t imagine that the FAA has relaxed any of the relevant requirements for emergency egress