Why do jobs - or careers - that require schooling and or specialized skills pay more than jobs that only require "on the job training?"

Paying for the investment the person made for education: Paying a higher salary aids the employee to pay for the investment they made in their education and saves the company from having to train employees for the job.

When a company hires a trained employee, the person can begin work immediately producing product for the company. They are being rewarded for the training they received and it motivates people to continue to get educated to fill those positions.