I've heard the argument that MMOs are only for interacting with other players since EverQuest in the 90s, when players tired of having to rely on groups for every single thing and just wanted to progress something at their own pace.
Then games like WoW and EQII came out. Both had a solo experience from day 1. There was still a point where you reached a solo content ceiling in them and would need to step into grouping or raiding, but it was a first step. The MMO genre has kept changing since that point, incorporating this idea more and more. Because developers realized the appeal was never being forced to interact. It was being together online in a dynamic and persistent world and having agency and choice in the way they interacted with others.