I'll say we call a leg a leg and a tail a tail because we can, because we speak a language which is complex, growing, and we like it that way.
In all other languages (I read this) there are far fewer words than are commonly used in English, and in some languages, they have to include English words to describe anything much more than basic concepts. As English includes many foreign words, (like all the words!) the difference is that English ADOPTS words, often for no good reason other than someone likes the word, and if we all like it, we use it and ascribe a meaning. Look up the origins of the word 'Quiz' sometime.

Leg a leg and tail a tail? We have LOTS of synonyms for jig a jig!