Why are hidden villages called such?
The literal translation for Konohagakure no Sato is "Village hidden in the leaves" (not hidden leaf village). Here hidden is used in the sense of "surrounded by" or "enclosed by". It does not mean those places are literally hidden from the world. It is similar for other villages, such as Amegakure no Sato (village hidden in the rain), Sunagakure no Sato (village hidden in the sand), etc.