This isn't true for all people online, but I think at least some men (and women) attempt to develop an online FANTASY romance with someone - a fantasy that satisfies their needs that are not met in their real world relationship. However, they have no intention of pursuing the online romance any further than the fantasy. They aren't interested in meeting the person face-to-face, and if their online romantic partner says or does something to disrupt their fantasy, they quickly end the relationship.
Using a technical psychological term, the online romance is a "dissociated" fantasy.