Most of the big names in the flerf community are grifters, for sure. But many of the people who follow them are genuine believers. I have a relative who's a die-hard flerfer.
Sheeps and Neeps was a small time flerfer who literally died because his conspiracy theory beliefs about cancer treatment, and there was that guy who went up in a rocket and came down in pieces. So some of them really do believe it. In my relative's case, he went through a lot of trauma, flerfers give him a sense of belonging and something to hold onto.