Muslims are trying to imitate the prophets and muhamad.
They go as far as they even try to look like them. You know, the classical Islamist beard? That's the reason.
Why is there no such thing among Christians? In Islam this is very mainstream, here no one would get the idea.
Do you try to look like Jesus or a Saint?
If not by a beard or hair, at least by jewelry or garments or whatever?