1/8 km = 0.125km = 125m = 12500cm

So freaking what. Use decimals and you can easily convert anything into everything. And you don’t have to rely on those few cases where you can exactly divide.

6.8m = 680cm = 0.0068km, etc.

Who writes out prices in fractions?

Its even worse with baking. Scaling up units from recipes is really hard if you have fractions of different units like tee spoon, table spoon and cup and when ounces can both mean a volume and a weight. (“fl. is not always specified”) No thank you.

Also your examples are really poor, when the base systems keep changing in the same dimensions (e.g. length) there is not benefit at all, just confusion or reluctance to convert to other units because it would be too awkward.