Hiring managers / business owners simply do not want sellers of labor to know the minimum they are willing to pay. Naming the minimum is a loss of negotiating position for the purchaser of labor, and that is why employers do not like the idea.
Employers also do not like the idea of having to increase their current employees' pay if market prices are rising. I know, because I have benefited greatly from paying different people different amounts simply because they do not know what I am willing to offer other employees.