Because it's something that most teenagers wouldn't want to have their parents look over and see on their screen, most things like this (imo) should be labeled as such, I'm kind of shocked by the number of times sexual and swearing things get off not nsfw, because really, do you want middle schoolers seeing that?
Nsfw stands for not safe for work, by the way. Also there are swearwords all over sites like these. If you are scared of your parents seeing a cuss word it would probably be safer to not go on sites like this.
I know that, and I'm 19, I don't give a shit if my parents see something, what I don't want is for my 6 year old cousin to be looking over my shoulder and see something that isn't appropriate, or for my 13 year old cousin to say "hey, my cousin goes on that site I should too!"
"Look at all the fucks I give"
Holy fuck