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.