It depends largely on the game, but the appeal of the genre is really in the scale of the games (hence, Massive Multiplayer Online), the freedom and the persistence of the world.
Most popular games can host less than 100 people in multiplayer. Fortnite and PUBG are 100 players, TF2 is Max 32 (16 v 16), Overwatch is 12 (6 v 6), Call of Duty lobbies are rarely larger than 32, etc. In addition to that most of the popular online games are Player vs. Player - direct competition with human opponents, which isn't everyone's forte.
For larger MMOS (WoW/FFXIV) you're likely to encouter hundreds of players in any city in the game, at any time of day, basically forever. You're likely to run into large groups of players in popular areas - much larger than any other type of game.
The game modes and the way you play it are much less restrictive than other games - you are much more free to play the game how you like. There's your typical break down of content, dungeons and player vs. player fare, but you're can just focus on crafting or shitposting in the chat if you're inclined. WoW (early on) had a reputation for being shitpost central (barrens chat). FFXIV in particular has a lot of modes/functions that are "just fucking around", such as Chocobo Racing/Breeding, Triple Triad, Housing, Majong, Fashion Report; you can even just hang out in town play music as a Bard if you want to. You can co-ordinate with players and do really dumb shit like make a band, co-ordinate a fireworks show, roleplay in a bar or something, troll or help strangers, etc - where you don't have that kind of freedom in other games.
The communities are also much larger - it's not uncommon for a guild to have in excess of 1,000 people (even if the guild has to be multiple guilds) where as other games don't really have such a tool, much less a reason for it to exist. There's nothing you can do in Call of Duty that would need ~60 people co-ordinated and it's a game that you really don't need to co-ordinate in beyond maybe using voice chat (if you want to).
Lastly, you never lose your character or progress in a MMO. Call of Duty starts you at the bottom of the ladder, level 1, every year (as it's a new game). When MMOS update, there's new things to do but you never go backwards so there's an appeal of that as well.