A lot of the time, though, we might imagine this is all two week's chat and then the begging starts, but these people are good at what they do. Sometimes they have spent a year or more sucking their victim in. Getting really close, building a relationship, getting someone to fall for them.
It seems obvious to the outsider, but I have seen programmes on this where the victim has been in constant internet conversation with the scammer for a very long time. Some say they were suspicious at first but as they got closer, they started to relax. They enjoyed time spent with Bobby Scammer. There was a valid reason they hadn't met IRL, usually distance/finance on both sides, or so they thought. Bobby Scammer doesn't push anything, gives plenty of realistic details about his life, and gradually gets his victim to the point where the 'relationship' is important to the victim.
Conversation then turns to serious effort to meet in person. Scammer knows the victim wants this. Perhaps Scammer invites the victim to come to America/Australia/wherever, knowing full well she can't because she's got X medical condition. She reminds him, he says, oh, of course. I'd come to you but money is tight, etc.
And then it starts. Victim is in deep at this point, will often have been forgoing friend and family time to chat with Bobby Scammer every night. If she wasn't vulnerable before, he's pushed her into a state of vulnerability simply by getting her to want to spend online time with him.
We all think we wouldn't fall for it under any circumstances, but when it's a massively gradual process, I can see how it happens. Not dissimilar to the charmers who are the perfect boyfriend until they move in.
When people have invested so much time in a relationship and become deeply in love, they aren't always rational. They might have been six months before when it was just a bit of flirty messages and they weren't really invested in it.
The warning bells might ring as the money requests come thick and fast, but the relationship you've had for a year can't all be lies, can it? You know there's something wrong, but you don't want there to be, because you don't want to lose the man who has been so perfect, so wonderful, so funny, he understood you so well, etc.