Well theres no reason games aren't fun, i play them everyday because they're fun. I play extinction for the sole purpose of mindlessly grinding alien hordes to a pulp because there is just some crazy amusement out of just shooting creatures. On top of that there's hella side challenges to make the game a little more interesting because you get bonus points for completing them, and who doesn't love a good bonus? i know i love a good bonus, especially when they help me dominate aliens even more. that's pretty fun to me.
Another game i love playing because its fun is the goat simulator. There is no point to the goat game, at all, but the content in the game is what gets me. In the game you're a goat who just goes around licking things and fucking shit up, along with explosions and funny little side events. This game is an easter egg based game i guess and honestly i love easter eggs, or finding things in the game that are off the beaten path, thats fun to me.