It creates an unhealthy attachment style that looks remarkably like drug addiction. Also, from a behaviorist perspective, the inconsistent "rewards" from an emotionally unavailable love interest sets up the same reward pattern that gamblers get hooked on.

Basically, it's hacking the basic human instinct to seek out love and affection. Emotionally unavailable people can exploit this "bug" in human systems, especially with people who are already emotionally damaged from their past.

Also, there's a seeming confidence in people who are emotionally unavailable. This lack of vulnerability is attractive to some people, especially those who haven't come to terms with their own vulnerability.