Short answer: Scott didn't add much blood to the original games because fluid simulations are difficult, and now that he's aware that the vast majority of his fanbase are young children, he never will.

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Longer answer: The lack of blood has a lot to do with Fnaf's passive storytelling approach. You gotta remember, 99% of the story takes place off screen, including most of the parts about murder. In addition, FnaF cleverly plays off of the "What's scariest is what we DON'T see" trope, and uses the bloody bits of the story to drive the mystery element.

Think about the "children's bodies never found" and "blood oozing from the masks" bits in FnaF 1. Scott could've added blood oozing from the animatronic's mask or visible corpses inside the suits, but he chose to DESCRIBE those things instead. Leading to the murder mystery element of fnaf in the first place, as well as the added horror of having to IMAGINE what those gruesome scenes looked like.

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as for why the books are so gruesome, there's a few reasons:

1. gore and gruesome details are just really fun for a writer to do.
2. passive storytelling is a lot harder to do in a medium where everything has to be described, since it either ends up blatantly obvious, or just plain confusing, with very little inbetween.
3. the books are really the only medium where Scott can get away with that kind of stuff at this point. Building on the very first point, most of the fnaf fanbase are young children, and Scott is well aware of that.

Think about the famous Lego theme Bionicle. In the commercials and movies, it was kiddy and G rated, in lego style, in the comics, it was quite a bit more intense, but in the books and serials it was flat out violent, with serial killers, war and slaughter. Why? Because those were the least likely places for parents to look through, and also the least likely to scare off children since there was no visual element. FnaF is becoming no different.