"Hey uh, there's nothing to worry about! Now, the animatronics may move around at night...." - Phone Guy, FNaF1
What are animatronics? You may be wondering. Now, you may think 'robot,' when I say 'animatronic,' but those are two different things. See, a robot is set to ONE task, although said task may be broadened. It cannot learn anything except that task and its surroundings. An animatronic, however, is either remotely controlled or made a set walking path. They can learn from their surroundings, the people around them, and anything near them.
In FNaF 1, 2, 3 and 4, we see nothing of a floor plan, and you are the last one in the pizzeria/house. So.. Why are the scary robots moving? Easily answered: Not only are they either programmed to move by themselves, or it's a glitch. It may also be added with the fact that DEAD KIDS are stuffed inside them. As such, it is asked that you must survive these kids. Seems easy enough, right? Wrong.
These ghosts have been around, pretending to have deactivtheyat night, when really, they're alive and vengeful, going after you, the night guard. You must try to stay alive for six hours, or else you will be stuffed into an animatronic suit. This might not seem bad, until you think about the insides of an animatronic. There's the wiring, the mainframe, the crossarms and of course, the material so stuff won't explode/catch fire/ glitch off/ stop working. This might end painfully slow.
Now don't get mad at the animatronics, now. That's not their fault. Here is the story of those seven dead children...
Twas once many many years ago. There lived a boy with his family, a mother and father, and a brother. The brother had some friends, and thēey tormented this poor child relentlessly. One day they'd gone to Fredbear's Family Diner, the place where the father of the boy owned. The brother's friends decided that since the boy had a fear of animatronics, to bring him up nice and close to one, named Fredbear. Fredbear and his good ol pal Springbonnie worked together as their own two-man band, ya see. The boy had his head shoved into the animatronic's mouth, which in turn snapped shut and closed on the boy's head.
Now for a solid week in the hospital, the boy kept telling people that there were spooky nightmarish animatronics out to get him, but nothing ever happened to change that. Eventually his head injure proved to be too much for him, and he faltered to continue on. His father suffered severely, and then he got angry with the children who had done his younger son wrong. He murdered them, even his own last living son, and stuffed them into suits. All six. Then the boy could not give up being on earth until he'd had his revenge, and eventually found a new form, as a tall but skinny puppet. He made those animatronics come to life with the dead spirits, swearing he'd never let them go until it was over and done.
Now do you see? It wasn't their fault. It was everyone and no-one.