Humans suffered colds without antibiotics for hundreds of thousands of years. Just because we can endure certain circumstances does not make them optimal. Our ancestors certainly didn't avoid softer sleeping spots because of some benefit the firmness of the ground provided.

Humans didn't sleep directly on the ground. Even in hot environments if the ground is even a little damp it will suck all the heat out of your body and chill you to death. What humans have always done is built an insulating mattress out of whatever material is at-hand and then slept on that. You can sleep on straw, dry grass, dry leaves, sticks, burlap or whatever you can find. But you never just sleep straight on the ground.