Wait as stupid as this "challenge" is do other drivers really just keep driving and run them over? I think they are equally as stupid. It's not like they pop out of the car into the traffic, they're on the road for a decent amount of time before they get hit. Plenty of time for any traffic to slow down and stop... and probably honk the horn and wave their arms in rage.
I don't think anyone is actually being run over. They're just stating that it increases the potential for serious injury. This is like the Teen Wolf car surfing, The Program laying down in the middle of the road, "ghost ride your whip" (actually isn't that the same thing as this?) type of thing where I think its somewhat being blown out of proportion (that's not to say some people aren't being stupid and doing this, often being injured lightly but most importantly setting up potentially catastrophic situations).
This is stupid, but its not been quite as overtly stupid as you seem to be taking it (they are not doing this suddenly in the middle of fast moving traffic, all the ones I've seen they were going slow and likely on low-traffic streets). I saw one where someone in the backseat got out and did it along a freeway, but the car was pretty clearly on the shoulder area (not in actual lane of traffic).
Also, believe it or not, people did stupid shit like this before social media and it wasn't just Millennials. I seem to recall the Top Gear guys reminiscing about similar stupid shit they'd do, like the driver climbing out the window over the car and into the other side. I think there was a "musical chairs" one that people did where they'd stop or slow down and then honk and everyone would change to a different seat too. And I know that loading up a pickup bed full of idiots and driving like a jackass was common.