Swimming

This is kind of random, but I recently read a news story where a 25 year old man and his 20 year old brother both drowned in a river because they didn't know how to swim. Now, at the risk of sounding like an insensitive asshole, how does one not know how to swim by the time they're 20. Even if you were never taught by anyone, I feel like a combination of natural instinct, embarrassment, and curiosity would be enough of a motivator to learn. Maybe this isn't as natural as I think it is since I've grown up around swimming pools, but I still think it's weird...