I think the argument for some of these licenses is that if the licensee screws up you can report them to a proper government agency who will investigate them. If a cab driver refuses service for a discriminatory reason in my town he can be reported to the the taxi cab bureau, or if my barber has unsanitary practices to health and human services.
And the barber example makes my point. When they added licensing for cosmetology in my state existing cosmetologists were grandfathered in. They got licenses because they'd already been doing the work. But if you wanted to enter the field you had to go to cosmetology school.
While the population in large cities is growing, plenty of people don't live in one. "Take the next one" simply isn't an option for those people (I speak from experience, having lived in the countryside).
(Somewhat tongue in cheek, but it's good to remember that there's more to life - and perhaps even potential markets.)