Of course they don't. tits gore and cursing is not what makes a good game, they're part of some good games, but not essential to games in general.
Some games, obviously will benefit for beeing aimed at an older audience, if the game actually calls for gore tits and swearing, don't cut it just to lower the age rating, if it will leave something lacking from them game.
Generally i thikn the age rating system is no god when it comes to gagueing a games "maturity". you can make an 18+ game full of everything that'll grant it an 18+ ratng and still have it be the most immature game in ages, and on the other hand, a good deep game doesn't neccecerily need those things to be well fit for mature players, and can easily get a low age rating.
Remember that the age ratings are not meant as a guideline to which age-group will enjoy the game the most, it's meant as a sort of security for preventing raging parents from attacking the game/movie companies cause their little kid leanerd some new naughty words.
As for the OP's friend, while this will never be more than guessing from my part, i'll take a shot at it.
I'll guess your friend, like many other people his age and older (you said teenager, so i assume thats what he is) are more or less obsessed with appearing mature and grown up.
They are afraid that people will see them as childish, and over-compensates for this by beeing very open for anything that they think will make them seem like adults, and generally brushing everything away that might make others think of younger children when watching their actions.
He'll grow out of it sooner or later, and learn that just cause something is enjoyable for a child, doesn't mean it has to be unenjoyable for adults. A good example of this is animated movies, like pixar movies and such, which a lot of young people will automaticly dismiss, cause they label them as childish. Ofc some of them are aimed more at children and might not offer as much to an adult, but others will offer way more to someone 31 years of age than someone 13 year old. Still cause they're essentially cartoons, they'lll be dismissed as kiddy things by some people.
Moviebob just did a "the big picture" episode about grim comics that relates pretty well to this, but from the perspective of someone who has lived too see a bunch of years more than i have.