Calling FNaF a cash cow is weird.
The first one was a runaway success that surprised Cawthorn himself. So he made 2 because of course. Then it kept getting more popular so he made more and got book deals and stuff.
But, he doesn’t always monetize them. Like FNaF World was a huge departure from the usual gameplay, being a turn based RPG. IIRC he charged for it at first, but because it was weak garbage he pulled the game and refunded all buyers. Then he made it good and released it for free. And later on he released another FNaF for free, but he did it without announcement and under an alt account or something as a surprise to people thinking they were playing something unrelated to FNaF from a new indie.
So yeah. The guy clearly isn’t churning out entries for easy cash. He does it because he just loves making the series and already has all the money he could ever need.