One thought is that when we engage with someone online, the information we get from him or her is low bandwidth and it is open textured, meaning that it is incomplete, vague, and ambiguous. By contrast, when we meet someone in real life we take in a tremendous amount of information about how they appear, how they carry themselves, how they speak, dress, etc. Electronically mediated communication carries a lot of information, but it is clearly impoverished compared to what we can get from first person contact. When the human mind receives incomplete information it doesn't perceive an incomplete canvass—it fills in the blanks. From an evolutionary perspective this makes perfect sense. We want to perceive predators and food sources in conditions where we have incomplete perceptual information. But in some cases we fill in the blanks incorrectly and in other cases we engage in a kind of wish projection.
I have seen the Facebook chat logs from one victim of Catfish-style deception, and to my eyes the record was surprisingly spare. There was just too much missing from the communications coming from the deceiver. Of course the victim didn't see it this way; he was filling in the blanks himself and that is what made the connection so powerful—the deceived could project anything he wanted, and he used the open canvass to project the kind of partner he most desired. Of course the hoaxer played along, letting the victim paint whatever picture he desired and affirming that picture at critical junctures.
But there is another point to be made. Let's suppose it is true that Manti Te'o was genuinely a victim of Catfish-style deception. What are the alternatives he was passing up in the real world? Were the potential partners crossing his path genuine? Were they seeing him for what he really was or for the brand that he had become—Notre Dame's big man on campus? We all want to be seen for who (we think) we really are, but there is no guarantee that meeting someone in meatspace and shaking his or her hand guarantees this.
I think this phenomenon played a role in the cases of deception that I am personally familiar with. In one case it was a beautiful woman who was being seen as just that by everyone who met her. The other was an attractive, exceptionally wealthy young man. Sometimes appearances can typecast you and online relationships allow you to break out of your traditional role. Thus, not only does the online world allow you to project your desires onto your partner, but you can also control how they see you. With a willing virtual partner you can engage in a collaborative work of fiction in which you are allowed to rewrite yourself as the kind of person you wish to be (or perhaps really are).