I never tried things like Warcraft, do you know why are they popular or addictive?

It's a complicated question, but a crude answer would be because they provide an easy and convenient activity structured so that you receive (psychological) rewards at a steady clip and because they provide a variety of rewards for people who want different things. Some people are loot-centric (and so they farm), some people are social-centric (and so they do things only with their guild), some people do PvP (some as a sport, and some as an exercise in griefing), some people enjoy exploration of the virtual world, some people like puzzles (e.g. how kill a boss with an underpowered character), etc. The relevant idea here, however, is that you leave the dull mundane world behind and submerge into a rather more exciting virtual world.

Social is an important part of it, and not only because of "social" aspects, but also because it makes the environment much more unpredictable, complex, and challenging. In single-player games you are usually in full control. In MMORGs you are not.