Why cant Zoro cut an opponents swords?

Logic?
Its something thats bothered me for a while now, he had the strength to consistently shatter his own swords as he fights, we've seen him lift ridiculous weights, throw a freaking stone house, cut through iron chains of ludicrous size, slice a ship in half (an omage to his teacher I believe) and now we've seen him cut through something akin to a mountain with seeming ease. Its pretty clear he does not lack the Skill or the Strength needed to cut something like another persons blade even without either party using Haki. Luffy even has BITTEN through steel seemingly the same width as an average sword. Id like to hear your thoughts, is it just out of common courtesy as a swordsman? Is it the want for a challenge? so his skills dont dull in actual combat?