I stopped questioning these things many years ago. I have no idea why it is that way. It simply is.

pic of Santiago non-con petting Louis’ hair bc of reasons
I think the vampiric parasite (Amel) analyzes the blueprint of the host’s body when it’s installed and it then immediately uninstalls the features it doesn’t need (e.g. internal reproductive organs). It then starts converting all the organic matter of the host body into its own substance to “perfect it” into the pure supernatural killing machine that it wants to be.
In that sense, that initial blueprint probably indicates length of hair, beard growth, muscle shape and position, etc., at time of death, and those are elements that the vampiric parasite program respects and wants to maintain as it continues to “update” its host body. It could be detrimental if a vampire shaved its head and then was stuck like that for eternity. The external appearance of the host body will affect its ability to hunt, and thus, preserve itself. It’s in the parasite’s interest for the host to continue to survive so that it can, too.
However, if the vampire had shaved as a mortal before turning, then that would be maintained.
BTW, about nails:
The nail growth part of vampire mythology seems to stem from how corpses tended to shrivel from dessication in their coffins after death, hence, making it appear that their fingernails had grown longer. Although I think hair does continue to grow for a short while after death? I’m not sure.