Way I always understood it is that the voice acting in SH games was done like that because all of the characters in those games (minus the evil-moustache level villains aka: Claudia), are just... normies. Regular folks thrown into nightmarish scenarios, and how I assume most folks would act.

That's part of it, they other part is that the creators really liked David Lynch and tried to capture the weird, offputting performances of Twin Peaks or Blue Velvet