MMO games are designed so that people can get together and socialize in an online environment. There has never been a hard and fast rule that people actually had to PLAY together, just that the environment is designed for people to BE together. The first MMO games made it mandatory for people to group together in order to enjoy most aspects of the game. Because this was the standard design for several years, many players developed a mindset that this forced group dynamic was the only way MMO games could, or should, function.
It turns out that most people simply don't WANT to be forced to group with a bunch of strangers in order to enjoy the game they bought. They DO, however, want to be a part of an immersive world where living players add a random and chaotic dynamic to the environment. They like to follow conversations that appear in the chat boxes. Some players even enjoy the presence of a guild, but still rather play solo.
I once considered the Elder Scrolls games to be the best in RPG gaming due to the massive and open worlds in each game. But there was something very lonely about playing those games. There was no one to show off too. No one to chat with. The NPCs were devoid of life beyond some scripted code. If a player came across a difficult mob and beat it, who do you tell? What if you find a rare item that has some awesome features, who does the player show it off to?
When I began playing my first MMO, I was first caught up with how lively it was. There was a sense that the world was populated, alive. The group content was a nice option but I didn't have to do it. My first guild was full of socializers, it was great.
This is why some of us choose to play solo in an MMO. Because we like a populated and living environment, but we don't want to be forced to attend the social gatherings.