Most multiplayer games don't have much of a sense of purpose, for me
With a singleplayer game, usually there's a reason given for the objective I am to complete, and completing that objective will result in some great wonderful thing like saving the world.
With a multiplayer game, things are usually more open-ended (see games like Minecraft) or more round-based (see games like Team Fortress 2, League of Legends, or Fortnite). Consequently, I have less of a sense of purpose, less of a reason to achieve a goal beyond just "winning". Winning alone is not enough to satisfy me. I want to know what winning will achieve, and who will it be achieved for, and what the results of winning will be. I want to solve a long form conflict, not a one-off round that starts over again.
Additionally, multiplayer games are consequently usually not story oriented, and even when they are, playing is dependent on the other player. For example, I was playing God Eater Resurrection, a coop monsterhunter-like, with a friend. We'd progress the story together completing the next mission in the plot. However now, because I was playing with them, I have to *always* play with them or they'll be left behind. What if they're busy? What if they feel like doing something else? I don't like being dependent on other people to play a game. I also have a short attention span, so likewise, if they wanna play and I don't, then that hurts them.
My favorite genre is the JRPG, and those are usually longform story-driven singleplayer games. I like the emphasis on story, being able to move at my own pace, the oftentimes turn-based combat...That sorta thing. I can pause and walk away anytime. Save and continue. And experience it all on my own.