Devils advocate- isn't this a bit like arguing peanut butter shouldn't be uniformly priced, but you ought to negotiate the price of every jar based on various attributes of the farm where the peanuts were grown: soil acidity, use of pesticides, as well as farmers grit and dedication? A jar is a jar, and ideally, a job is a job. An employee can either do the job, and be paid what it says on the can, or they cannot do the job and have to be transitioned out.
Sure, jars come in different sizes and brands, but we should stop pretending each job is bespoke when employee organizations function on the basis of them being similar, empirically workers are replaceable.
My biggest problem is information asymmetry: even if I were to accept your argument at face value (that important attributes are hard to gauge); the employee has much less knowledge on how much they ought to be paid for their skill-level, but the employer will have that information up the wazoo (both internally, and through industry surveys and back-channels)