Trust that this was not at all FNAF's fault. People making stupid as fuck theories that only highlight how bad video game writing is has been around since Zelda. Then MatPat came around with his terrible EarthBound and UnderTale theories and that exploded it further. But this desperation to find something deeper in things you have to be high to think is astute coherent literature has been a trend for decades.
Oh for sure, it just feels like some devs run with and embrace that kind of stuff now. Whereas before, like with undertale for example, it wasn't designed specifically to be vague at any point I don't think. Like you said though, people like to assign grand, complex, and deep meaning to just about every little rock and pebble. And I still think the first few FNAF games weren't meant to be anything more than a fun experience with a few things sprinkled here and there.
But... some devs embrace that hunger for deeper meaning, highlighting everything that's slightly out of place and saying 'see that? see that? it's important too!' and again, never actually addressing it or anything beyond showing something equally vague following it later on.
I guess I'm just salty about the laziness of it and the obvious cash grab aspect. You know hello neighbor had a TV show pilot, books, and plushies? Gotta read the book if you wanna understand xy and z! Guess they thought that was more important than actually polishing their game...
Part of me thought Death Stranding was going to do the same thing when I first opened that one up... lol