These are smart people, they do not want it to change. Their income would decrease if more people were trained/brought in. Have a lot of physician friends, it's awful and they complain, but are worried about wages. They do not want to make less.
A big part is the expense of undergrad+med school+low residency wages+most grew up upper-middle class and have big expectations. They're very worried about losing ground wage-wise. which is fair in my opinion...they have a de facto union/guild (like most smart people who earn a wage try to do).