Video games that are only fun with friends and not on their own are generally doomed to a flash of success and then death. See: Among Us in like 6 months, or any of those party games. You might pick them up in a year or two for like, an evening or a week, but you won't exactly be coming back to it on the weekly. 

That's a horrible model for an MMO, since MMOs rely on replay value to make their money. Games like Among Us and Town of Salem and all those other "fun with friends but not on their own" little games are one-time sales - the company has made their money whether you play it continually or not, and in that sense can be called a success. But since the MMO model is too long-running to rely on initial sales alone, they NEED people to keep coming back (and frequently, too) in order to stay profitable. And before you come back screaming capitalism, you should definitely care if your MMOs make money, cause if they don't, they die and you don't get to play them anymore. 

Besides, it's nice to be able to play an MMO you enjoy without [i]needing[/i] other people, for those times when you want to play but your friends aren't around, or it's a slow time on the server and queues are taking forever. Engaging singleplayer content (especially overarching main story) is also a great way to get players who haven't played in a while to come back for updates and expansions, because the singleplayer content has them engaged enough to want to see what comes next regardless of if their friends still play the game. 

Singleplayer content also adds to the multiplayer experience. An MMO that's basically just a chat room relies on the players to make the content, and they're not always up to the task. But an MMO that also has a main story going on and side content that's done singleplayer can enhance the multiplayer experience by giving you and your friends something to talk about, compare, or even turn into a multiplayer engagement (ex. gathering convoys in XIV, housing, etc). 

Then there's kinda hybrid content where it's singleplayer 95% of the time but requires multiplayer objectives to progress, ex. story quests in XIV that are played singleplayer until you reach a dungeon or trial requiring other players to beat. Mind you, that's [i]hours[/i] of singleplayer content combined with maybe a combined hour of mandatory multiplayer across an entire expansion. 

Basically if I can't play the game unless my friends are online, then I probably just won't play the game, and that's a [i]terrible[/i] idea for the MMO model. 

And needless to say, MMOs absolutely have to be fun as games. "Facilitating social interaction" is the design philosophy of AA, not video games XD If it's not fun, it'll just be passed over in favour of the millions of other games that ARE fun, and therefore die. 

For the record, I love the multiplayer aspect of MMOs. I love playing with friends and even strangers. But if not for the singleplayer content I definitely wouldn't have stayed with FFXIV for this long. I would never have made it through the early leveling stages, since back then I was too baby to participate in the content my friends were running!