For the same reason big hair can make someone look crazy. They make you look unkempt.

Because you have a subconscious preconception about beards.

It is true that beards can hide expressions, making it harder to read someone, which can reduce “trust”. But that’s only part of it. Your idea that it makes them look crazy is all about your ideas of things.

You’re probably basing it on the unkempt nature of people that aren’t able to function well. The homeless, people that are “crazy” or otherwise behave unusually, that kind of thing. You’ve developed a connection between two things that aren’t really connected. An unkempt beard only means that the beard isn’t being grown to be socially conforming. The fact that people that are unable to socially conform rather than choosing not to tend to be less capable of maintaining a beard to social standards is a separate issue.

Don’t get me wrong, it’s a fairly common thing to have that preconception. I have a big ol beard and sometimes catch myself spending extra time evaluating the threat level of someone that’s ragged.

Don’t worry about it. We all have preconceptions, it’s how human’s minds work. Just keep yourself aware that you can’t reliably trust preconceptions.

Big beards can alter facial recognition and obscure expressions, making someone look more unpredictable or wild to observers.

Currently travelling and managed to get my beard in check almost a week ago. Will probably be untouched for another month. My hair is also getting long. I’m afraid of weirding people out but every time I get a haircut and beard trim + wet shave they always make my beard too short and it’s annoying to grow it back to how I want it. I generally keep it reasonably short and neat but let’s see what happens after this next month of travel.