the single player ones are bad because you get bored of them pretty easy
A game being single player isn't what makes them boring; it's the lack of objectives, quests, adventures, etc. that makes a game boring. Take Minecraft for example. If you can't (or won't) make objectives for yourself, you will get bored quite fast due to not knowing what you should do. Obviously, it's a little hard to compare Slime Rancher and Minecraft, but the point is that neither of these games have clear objectives, nor are they linear in terms of how you reach a goal.
As for games like The Elder Scrolls franchise, they do have clearer objectives, but they are also not always linear. Plus, in order to start most quests, you have to either get lucky or go out of your way to do the quest.
You've also got to remember the play style of Slime Rancher, and how multiplayer would, a lot of the time, make a game that is usually calm, low stress, and how you can just put on a podcast or music and play without focusing too much on one thing. Oh, and it would take a lot of time to rebuild the game to support multiplayer.