Falls can happen so fast (front wheel giving way on slick surface, for example) that there is not time to react by putting out the hands. I came off on a corner that was damp, coarse chipseal and sprinkled with diesel from a bus. Landed on my right shoulder and didn't put my arms out at all. Dislocated the shoulder, but no road rash to speak of. Weird from that point of view.

If you look at the pros, often when they go down they are still holding on the handlebars and they are still clipped in to at least one pedal with the bike on top or beside them when they stop.

It's the low-speed tip-overs that can cause more pain/damage, in my experience. Quite a while ago, a curb hop went wrong and I went over the bars, putting out both hands to brace the fall. I though I had broken both wrists, but fortunately didn't. The bike ended up perfectly balanced on the handlebars and seat!