The only people who do it are the ones who've never fallen off without the right gear on. Once they do, they usually learn. If they survive/ever return to biking.
For most people the odds are so small that they'll have an accident whilst not wearing the right gear, they take the risk and they get away with it. Just like the guys who seem to know it's safe to overtake on blind bends, or weave in and out of rush hour traffic at 100mph+ because they "know" what the other drivers/riders are thinking
They'll learn...