It’s a question of Return On Investment (ROI).
For an established professional, the company gets positive ROI because of the quality, relevance, and volume of work produced by the individual. Therefore, the ROI merits paying the individual commensurate with his or her performance.
For an intern, the company trains the intern while receiving very little (if any) usable work product in the profession being interned for. Therefore, the amount of investment that is merited by the return is much, much lower - and in some cases is (monetarily) nothing.
And the company would get an even great ROI if they just didn’t pay their employees at all... and simply hired a few guards to keep them from leaving.
Chinese firms have figured this out. How could we even consider hindering our company’s ability to compete?