It's the standard for distances involving geography and motoring. Speed limits on the roads are and have always been in MPH. A car would be described as having a top speed of 150mph. If I describe a distance I would say London is about 100 miles away from Norwich or the post office is half a mile down the road. We turned metric sometime during the 1970's but not entirely, apart from with money. Small distances are described in either cm or inches, pretty much interchangeably. The height of people is almost always described using feet and inches, hardly ever metres. Likewise, the weight of people in stones rather than kg or pounds.