Imagine how painful it is to be an entrepreneur with a very weak signal about how much you can pay people to remain happy to work with you without going broke and still retaining good enough talent.
So the way to counteract information asymmetry (if we assume it exists) is to artificially enforce less information on the employers? That can't possibly be the right answer unless there's some systematic bias that leads to worse economic outcomes when employers happen to know a candidate's past income.
Large and medium size businesses have much more information available to them than their applicants do.