In many SOF kayaks the paddler keeps their legs flat and straight so they don't need any extra support under their thighs. Also the skin supports the butt comfortably. Using a pad along the bottom of a SOF provides some extra comfort, helps hold any dirt or sand from your feet and allows an easier enter and exit of your feet as they go by the ribs.
One of the things I really like about a SOF is that your center of gravity is lower and provides a noticeably greater amount of stability from the exact same hull. It still blows my mind why manufactured kayaks install seats that are 1"- 1 1/2" off the bottom of the hull. The only reason I can think of is that most manufactured kayaks are expedition size kayaks with high decks, and maybe by raising the seat, the paddler doesn't have to reach so high over the deck to paddle.