And also, unlike the weirdness of pre-decimal currency or some of our more obscure weights and measures units, the miles we talk about are rough approximations

And also, unlike the weirdness of pre-decimal currency or some of our more obscure weights and measures units, the miles we talk about are rough approximations that aren't supposed to be subdivided or converted into anything precise (or anything more precise than "about half a mile") so there's no particular advantage to using an SI unit