Even when people disbelieve in the ghosts; it should be easy enough to prove that their equipment does stuff. Each Ghostbuster wears an "unlicensed nuclear accelerator" on their back that can clearly and undoubtedly send out a stream of energy capable of intense physical damage. ....yet nobody else in their world seems capable of, or even interested in duplicating this technology. This is because they can't. It only works because Egon and Ray believe it works. (Peter and Winston don't care how it works, just that it does, so they don't interfere with the effect.) Any attempts to reproduce this effect would either just fizzle or explode.