Well character make up requires some talent and thought and often actors (and I use the term loosely to include scaracters) aren't "acting" per se but rather scaracterizing. They go for the carnage "eeek" instead of the more psychological "thrill"
Most characters require some depth (history or background) to them and unfortunately in a walk through haunt it's hard to develop a character relationship with the audience within a few seconds of exposure. A character would need to be highly integrated to his set (environment) in order to tell the story simply by appearance. He may also need to work the outside line before the guests encounter them inside the haunt to fully exploit the psychological impact of their character.
Then they are acting....if they can emote through a character and relate to the audience they will get a much more effective performance.
That's why I prefer make up or prosthetics to masks. An actor's only tool is his face and if you put a mask on an actor you stiffle any creative opportunity he has. An actor uses make up as a tool to REALize the character he is trying to develop.