Supply and demand is a factor but it is not the only factor. The reality is that AMC's exploit supply and demand to obtain work at the lowest fees, whereas direct lenders and non AMC/private work view supply and demand to obtain quality work at a fair fee.
Since the focus is on mortgage lender work, look at the difference in fees between AMC's and direct lenders in any given region. Typically, the AMC fee is substantially lower. Why is that the case, when the borrowers are paying equivalent appraisal fees in that region, and supply of appraisers is the same in that region? It is because AMC's have a specific priority in assigning orders ( lower fee). Direct lenders who are not profiting off of lower fees have no incentive to shop by lower fee. As long as what the appraiser charges is covered by what borrower pays, it's good.
To sum up, in any given region there are two sets of fees being paid with same supply number of licensed appraisers present. AMC fees, and direct lender/non AMC fees. The stark difference between the two is apparent . The fact that only in 3 states see AMC rates and direct lender/non AMC rates equivalent, only shows that supply/demand has to reach an extreme imbalance to coerce an AMC to abandon their tactics to hire at low fees. A severe imbalance as the only way to get a fair fee for professionals is not how supply and demand normally works- in most professions, there is normally an ample supply of professionals, yet they still can command a fair fee for service..
It is not healthy for the appraisal profession nor for consumers and users to have the only way AMC's paying a fair fee is for a steep attrition of appraisers occur causing a severe imbalance.