Because, it can be played even when you're home. It's that efficient. I guess that answers your question.
To compete with other players, its boring to play alone all the time. Also it saves cost of downloading the game.
I've never really enjoyed story modes myself. I like playing games online for the competition, team work and I also tend to play games that my friends also play, so we can play cooperatively.
I think, because people just in front of PC and online game play is also the same as the regular game.
I play online games because I am very competitive and I have met a lot of great friends over the years playing games online. I even met my wife playing a game and we have been married nearly 5 years!
I like the competition of playing online. Especially when playing games that constantly have new circumstances like League of Legends. Every game you're playing with new players, new champions and new strategies.
I would imagine it is because of the competition like many people have mentioned. Playing against real people is a lot more challenging as they can be much more creative. I think the social aspect is a big part of it too.
Online Game for always presenting a new type of game, and we can play with friends / at a time when the same friend