I think a lot of ultralight items are jugdged by measurable properties primarily (which makes the whole subculture very nerd-compatible, I guess). Ofc that doesn't mean you don't care at all for things like comfort.

A solid pad in Ultralight often also doubles in the backpack as a replacement for what would be the frame in traditional backpacks, so it's dual use (or triple, if you consider using it as a sitting mat when taking breaks.

Also, sleeping bags typically compress under your body and lose isolation because of that, so with a pad you can use quilt-style sleeping bags (they wrap around the top and sides but not bottom, which makes them again lighter).