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.