It’s a really complex thing I think. I’d guess it’s often attention, sometimes there may be monetary gain, or maybe they really are convinced there’s something wrong with them.
On the flip side I knew someone who was very sick with a few conditions and someone else (who I suspect had big issues of their own) decided they’d accuse them of being a faker. It was truly horrific the impact they had. And words are like toothpaste, you can’t get them back into the tube. So I’d be very careful around these matters myself.