In my 40 mph low-side on the left side almost a year ago, the bottom portion of my First Gear overpants got totally shreded from sliding and I had a nice rash on my left butt cheek. That area didn’t even seem reinforced. When I looked for a new pair of overpants I wanted better abrasion protection in the bottom area. I got Rev’It Turbine overpants which looks thicker in the bottom area, but I’m still unsure how it would stand up to a slide, so I’d like to get leather pants which are supposed to be the best for abrasion protection.
My theory on why the seat of the pants are not padded/armored is: a) it would interfere with you seating comfort, b) like others have said, the “impact” points are usually joints like elbow/shoulder/hip/knee, and c) there’s a lot of meat in the bottom so impact protection is “less critical” than abrasion protection in that particular area.