I don't understand this either even though I know it's most people's gaming habits now. Playing the same multiplayer game for years on end. To me, that's like watching the same movie on repeat or reading the same book over and over. I want new experiences.
I think it mostly has to do with the premise and the gameplay loop. Like, people play the same sports with basically no changes for how long now? The same card games, the same board games, etc. New games get made all the time but depending on where you live there's always go-tos like blackjack, ludo, go, checkers, dominoes, dice poker, backgammon, etc. Some modern people play 'games' on their phones, be they word search puzzles, crosswords, color-match, sudoku, or any of the variants of classic mobile game crap. Even I have the shitty Hill Climb Racing game on my phone, for the same damn reason.
It's there, you know how to play it, it gets the job done.
That's the casual mindset. The driving desire is just to get entertainment out of a game, pass some time focusing on something that isn't the real world, and maybe hang out with other people if it's multiplayer. With competitive titles it's somewhat flipped, the people who play those are very particular on the workings of the game, but the effect is functionally the same - they play it because they know how to, and it lets them get their entertainment easily exactly because the game doesn't change. They like to play by a particular set of rules, and thrive on being, or getting, better at it than others.