According to NIST special publication  the choice to a certain extent arbitrary because it is not essential to the physics of the subject andby convention are regarded as dimensionally independent Which is to say that they just are  and you are not to ask questions You are poking here at the rather tender underbelly of the SI The units are not as you point out in fact dimensionally independent however we wish they were and keep them seperate to allow for accruate measurements and convention Another sore topic is temperature Why are there two systems What sense does that make Its just like the tradiational system of fahrenheit except that the afine transformation is simpler and people liked it so they held on to it bc it wasnt that bad