Probably one of two reasons, either to scam someone or just because they wanted the real thing but couldn't get one. I like to think it's the latter.
It reminds me of this lad I used to know who was into kit cars, he'd buy old Toyota's and put Ferrari body kits on them, they even had false engine covers in the back and faux buckskin upholstery inside, he didn't do it in the hopes of fooling someone into paying Ferrari money for an MR2 though, he did it because he loved Ferraris and knew he was probably never going to be able to afford the real thing.
Personally I don't get it either, it's like wearing an imitation Rolex or fake designer clothes, I would know and so I wouldn't enjoy these items, the thought that some people might mistake them for the real thing doesn't appeal either, if a person said nice watch or nice car when It was fake I'd feel like a fraud and a liar, but people who do it for those kinds of reasons alone aren't really hurting anyone, those who do it with the express intention of fooling people into paying more than the item is worth are no better than thieves in my eyes.
I'm one of those people who fully accepts that Lamborghinis and Bugatti's (and Cunninghams and Kleins) are completely out of reach, and that's totally fine, I'm a blue collar man and I like my blue collar life and as much as I appreciate quality craftmanship and beautiful design there are plenty of things that tick those boxes and are available for me to own, appreciate and enjoy, like great old blue collar bikes.
This bike was also outed on one of the Facebook groups last week. I pointed out at the time it's not a Fat and my guess was also a Saracen. I hope people do their homework, or ask, although I have seen a couple of times recently people spending big money based purely on the sellers description, which was wrong.