Because to convert from one to t'other, you have to multiply by 22 x 10 x 8 or whatever it is, rather than simply shift the digits.
For example, there's a park that's 1 mi 180 yds from me - if you were to pick one and stick with it then you could say either 1.1 mis or 1940 yds, rather than mixing the two and expecting people to do multiple rounds of arithmetic.
Could that be a UK thing? I wouldn't know tbh, because between mixing yards and miles versus sticking to imperial units in the 21st century, the latter is definitely crazier ;)
In Canada we have imperial for short measurements (inches and feet for lumber, people's heights) but metric for longer distances (kilometers for driving distances for example)