(Yes. This one is an opinion. It's just my own thought of why Playtime Co is such a dangerous place to even be at in the first place.)

The popular and loved game Poppy Playtime. I am a huge fan, myself. But why is the factory Playtime Co so cursed with giant or small murderous toys? Or even very few toys with no intention to kill you? I don't think it's the fact that they were experimented on or can suddenly be brought to life. In real life, that wouldn't happen. Just saying. Not to be skeptical... about that part.

Something tells me that the reason this has all been happening is creepier. The toys going on a killing spree. The hundreds and hundreds of body counts of the employees. This is some curse, alright. 

Perhaps maybe the factory itself being possibly built around the 50s or 60s or before that... was actually built on a burial ground. A cursed ground. So, anything that gets built, buried, etc. on that particular property is bound to lead to something terrible.