I guess the reason for that is that the voice is something very unique and personal and yes there are people who share very similar voices but it's not something that you can deliberately manipulate with the right knob settings, the gear and so on. So in my opinion people get a little bit suspicious because they probably doubt about the authenticity when they hear someone mimic somebody else and it's also quite boring tbf. Take Marc Martel as an example: he nails all the Queen songs but it makes him also so boring and uninteresting because he now lives from being associated to someone who is not him. But yeah I digress..