That maybe was the case with the oldschool MMOs (Everquest for example) where you could do not do anything alone but nowdays you can pretty much play them as a single player game. FFXIV, WoW, GW2 etc. are incredibly solo friendly. You don't even need to join a guild or such there is automated matchmaking for most group content. You can play these games without ever interacting with any other players. And a lot of people do that because they care more about the world or story or things that don't need other players (see crafting, gathering etc.)
Heck even FFXI which soon will be 20 years old is solo friendly thanks to the NPC party members they introduced some years ago.