They were created well before the natural units or atomic units were discovered so they are going to based off of arbitrary measurements that were repeatable by

They were created well before the natural units or atomic units were discovered so they are going to based off of arbitrary measurements that were repeatable by scientists of the past. SI is just as arbitrary as imperial but at least it has the advantage of base-10 for the prefixes. The problem is that you can't just change the kilogram to whatever nice natural unit you want overnight so even moving to nice universal constants would still give you some weird coefficients.