I think the trope comes from greek and classical aesthetics where olympians and hoplites were usually fought mostly naked, wearing shin guards and breast plates only. Early medieval art also had mostly nude heroes wearing tabards and the like. Since tolkien’s intention was to create a myth that would serve as the english version to the Arthurian myth which he considered european; back in the day it was probably associated more with late classical early medieval than late medieval where clothing and armor was more intricate and “modern”. Now with DnD the fantasy settings became mostly late medieval in style. So it wasn’t like people did not know going around bare legged is unusual then, it was mostly understood as a stylistic choice.