Azuriel posted Revisting Single Player MMOs about how it is not just valid, it's common that people play MMOs as single player games. This made me realize that many people wouldn't play an MMO only one night a week and question why I'd want to do it.
I think it comes down to three things:
Longevity - A healthy MMO is constantly growing. There are a lot of things to do and there will be more over time. Most of the single player games I've played have either been RPGs that have a definite ending (e.g. Ultima games) or strategy games where you have variation between games (e.g. Civilization games). I find PvP too stressful, so, in either case I tend to lose interest within a couple of months. With a MMO, I can play it for years if it's maintained.
Investment - As I play MMOs I invest a lot in the characters I build up and I'm curious to see how he or she develops and interacts with the world.
Community - It is odd to include this when you are playing a MMO as a single player game, but there are two aspects of community that are still there. Even if you are just questing or exploring, every once it a while you can help someone or someone may help you. I have very low expectations for others in games, so it's always somewhat surprising when it happens, but it is nice when it does.
Finally there are all of the blogs. I enjoy reading about MMOs. Because MMOs are persistent and shared, they become a great source for discussing what people enjoy and why.
I have a lot of undeveloped ideas about why MMOs are enjoyable in them selves and in the broader community that I'm hoping to develop in this blog over time.