I don't think most of these think it's "fun", just that it's humorous to a lot of people, hence why it's also in tons of movies and TV shows. Then some people in any group of friends will be "the funny ones" and will want to go to any extreme to make their group laugh. Plus the fact that these people don't see v* as being as extreme as we do.

Also, with the milk/cinnamon ones, since they are "challenges" I'm assuming part of the aim is to beat the challenge. Probably some of these people look at whatever it is and think "of course I can ingest this, I've ingested way more" (or way worse, if it's something gross).