"capelli" is gramatically plural. I guess you meant semantically singular. That depends on your point of view. As "capello" (singular) can mean a single hair and the hair on your head has several hairs, referring to the latter as "capelli" (plural) does not look an idiosyncrasy to me, it is just different from English.
FYI French and Portuguese may also use plural to designate hair (over your head).