Sadly, for some people, those two lives merge together with each other. I may be wrong, but for some, that is the only life they can live.
Almost all of the folks you talk with online, you may never see in real life, so don't dwell upon it.
EDIT: Besides, the only people who deserves physical harm online are internet tough guys who talk a lot of shit online, but are general pussies and cowards offline. Still, like I stated above, you should not dwell on that.