Because they are lonely, mostly, and scammers say things they desperately need to believe in. Things like, they still have value and someone cares about them. When the scam falls apart, as they eventually do, aside from the loss of whatever the scammer wanted (usually money), the victim is also faced with the unbridled reality that they aren't valued and they don't have anyone who cares about them. It can be quite a shock to face the truth, even when one knows it in the back of their mind. Facing it and having to evolve into a person who is entirely self reliant is a real challenge for people who are prone to be catfished.
