Measurements can be combined with things that are already irreducible. For example, in baking, a recipe using “one egg” and “2 cups flour” has to keep the “one egg” if it’s going to convert to metric; and given conversion tables that favor nice round numbers over exact amounts, this means the proportions will change ever so slightly. It definitely doesn’t matter in every case but it’s these kinds of issues that would prevent people from wanting to spend time just converting everything.

I am a little surprised that the U.S. hasn’t adopted more “low hanging fruit” though. For instance, street signs could clearly display kilometer distances and km/h speed limits, especially if the “km” was included on the sign (as it was in Canada during a transition phase).