Because mixing units but keeping coefficients 1:1 means there's no math, as long as you measure your bodyweight in pounds and have a way of measuring your protein in grams (which food packaging does, even in the U.S.).

Mixing coefficients instead of units means you need to express it as 2.2g/kg which means having to do a calculation, or two calculations if you need to convert your bodyweight from pounds to kilograms.