But
it isn't
just that?
This is pretty tired piece of rhetoric, but, jumpscares are a tool to use for horror and FNaF uses them in what is probably the best way you can. The jumpscare isn't there to simply catch the player off guard, it's a punishment for losing.
This is what I think 1 and 2 were the best, they utilized this concept so well, maybe better than 4 and definitely better than 3. See, each night is just one long string of tension, tension that comes about due to the precise defensive nature of the gameplay mechanics, that can end either 1 of 2 ways. 1 is that the player makes it to the end of the night and the tension is resolved through relief, which is positive reinforcement. 2 is that the player fails and is punished with a jumpscare, which resolves the tension through surprise and very small heart attacks. The player knows the jumpscare is coming, and being one of the few ways in which a game can actually effect the player themselves, even more tension is built up knowing that that is what'll happen if you mess up. The jumpscare is actually used to amplify fear and tension, not just to get a cheap scare.