Everyone likes to be liked. Flirting is just another way of feeling like someone likes you, even though you know for a fact that it's meaningless, "just for fun" flirting. People just want to feel that attention, no matter how false it may be.
I think it's easier to "get serious" because you don't have the same hang-ups. Many people get very nervous and go quiet when in person, and just don't have the confidence that they do in an environment where they're not face-to-face.
As others have said, it's all about being open, and knowing where the lines are. As long as you understand that it's not real, you can flirt to your heart's content. For example, I am a pretty good flirt (though I think it's just cute, cheeky flirtation, not the type that would earn me the title of chat lothario or anything), and many of you know that. I don't, however, have any intentions with anyone other than hanging out and having fun. I can sit in laps, give massages, talk a little dirty, whatever. Just normal, flirty stuff. I flirt, in real life, with girls I know well, and who have boyfriends. In front of the boyfriends. They know it's innocent and harmless, that it means absolutely nothing, and it's just fun (and funny) all around. As long as the lines are established (or inherently known) and kept by both people, it's all good.
And now if you'll excuse me, i'm going to go grab some ass in chat.
See you there.