The only way higher wages lead to more candidates is by taking them away from other companies or other industries, and sometimes from other countries. So that is sometimes an option for certain employers, but not for an entire industry. It's a myth that there is a pool of workers who is "underemployed" because of low wages.
Also: In a free market, if companies can earn more profits by hiring more people by offering higher wages, they will do so. But increasing wages also reduces profit, and there just may not be enough room in the margins.