Why have they never taken the killers mask off… (unless they did and I’m forgetting)

They tried too a while back but its just kinda stuck on there

As far as I remember, I think they once tried taking the mask off from one of the killers and it turns out to be an ordinary person... something like that and it will spawn another different killer. Idk if I remembered it right or they didn't got to see the face but yeah I may be remembered it wrong