They aren't always. Some games offer comic mischief. Some games test your problem solving skills with puzzles.
If you're asking why violent videogames have to be so violent, it's complicated. You can either have a brutal depiction of good vs evil, a fighter trying to prove he's the strongest ever by executing his competition, or a sociopath doing whatever he pleases.
It depends on the target audience.
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I am sick, and in so being I am the healthy one.
I know what I hate in life and I love the fact that I hate these things in life.
If my darkness or eccentricness offends you, I don't really care.
I will not apologize for being me.