Deer sleep outside without tents, without pads, and without sleeping bags. Their summer coats are often sparse. They sleep well, even though the temperatures can vary quite a bit, and even though it is sometimes raining at night.
Granted, many of us humans are relatively hairless mammals. However, there are all kinds of substitutes for a coat of hair. Various wicking, warm-when-wet fabrics and wools are among them.
Deer often bed down in a little scrape in the forest. Bears do something very similar.
Our close relatives in Africa (various large primates there) also sleep under the stars in simple nests, often made quickly from leaves and twigs.
I once tried it while on tour, just to do it. One night I just slept on top of a big rock out in a lava field. I was able to get comfortable, even on a very hard surface, by positioning my body somewhat carefully, and finding a slightly sculpted indentation.
Human beings also have rain gear. And layering systems.
Why not sleep in a good rain suit? It simplifies things enormously.
It seems that this idea could be taken further. Maybe someone on this forum can help forge some new possibilities for simpler, more minimalist sleeping arrangements.