Exactly! It's because the official legal measure of distance is still the mile. (Don't ask!)
Joe P must have been in Eire, where they ran both mile and km signs for years (surprise, surprise....). Only recently is everthing in km.
In some Continental countries there is still an 'idiomatic' pound (500 g) and so people might speak about a half pound, meaning 250 g. There are (or were) also superseded monetary units such as the French sou, 5 centimes (this may have gone with the arrival of the euro.). None of these have any legal status.
There is some merit to Joe's point about having a single unit in an economic region, reducing pressure on US to change, but I think it is not a strong case. 'Europe' changed to fairly uniform metric measure in the 19th century when Napoleon occupied large chunks of it. This long preceded the antecedents of the EU (Treaty of Rome 1957).
And, how do we define a 'sufficient' region for standardisation? How about the globe? There imperial measure is hardly found. Just legal in mainly (only?) the USA and understood in a few other places. (The mile in the UK being one of those exceptions.)
There have been many arguments about the merits of the duodecimal system v decimal (and metric) and, while the duodecimal sytem does have a number of advantages, decimal/metric wins out for the vast majority of people.
DAS
For direct contact replace nospam with schmetterling