Basically you made the case for the metric system! Yes, sure, technically it's not that hard to convert between units, when all you're doing is focusing kn converting units. But in real life you're supposed to use your brain to do other stuff and if you have to stop and suddenly perform a contortion to compare the price per weigh unit of one item with another item that uses another base weigh unit, well, that's now an extra step that you didn't ask for.
With the metric system it can happen that some products use grams, and some kilograms (some countries use hectograms while others prefer decagrams, for some uses like food) and you have the same potential for inconsistency, however since there are only factors of ten at play, it doesn't take many brain cycles to quickly compare the numbers in your head.