One of the things are: Playing mp with friends is one thing, playing with random people is another and it can be a terrible experience that goes from awful team mates to a toxic environment.
Most gamers that dislike mp games simply can't play with friends most of time (life gets in the way), so they rather play SP games that are sure to entertain them.
MP only games are repetitive (yes, each match is different, but the actions are the same and the stages too), if the gameplay loop doesn't satisfy the player enough, there's no reason to play it, while a SP game can hold the player attention in multiple ways (story, characters, customizations...) and has the assurance that there is an end to the game, while the mp only games are basically a loop with close to zero progression.
Finally, most older gamers got in to video games not just because of the gameplay, but because of the interactivity, exploring worlds, making choices, creating characters and "living" a story... Most SP games (and MMORPGs) are a combination of these aspects, while games like Fortnite or Apex Legends are mostly and primary just one thing, one single loop over and over and over... Which is fine... But eh..
A dumb analogy in why I respect sp games a lot more than things like fortnite:
MP only games are like eating fast food, you might enjoy it, but is just cheap unhealthy fast food that is not memorable.
While SP games are like eating in a unique restaurant, the food might end up being terrible, but is a different experience so you will remember one way or another.