Why should specific frequencies be assigned for specific uses anyway? Can't we have a marketplace for spectrum in which devices can get access to the most appr

Why should specific frequencies be assigned for specific uses anyway? Can't we have a marketplace for spectrum in which devices can get access to the most appropriate spectrum in realtime? Spectrum is expensive because we waste so much of it. If users had an incentive to use the least usable frequencies when they do work, there'd be more room in the most usable frequencies for applications that require those. As it turns out, the FCC has approved sharing rules for the 3.5 gigahertz spectrum. Maybe this will demonstrate how much better the future can be. http://www.rcrwireless.com/20150515/test-and-measurement/isart-2015-google-spectrum-sharing-3-5-ghz-tag6