Yes. The carriers currently certify and subsidize devices for their network that are designed to work only on the spectrum that they own. This creates customer lock-in because (i) devices are expensive and (ii) if you want a subsidy, you sign a two year agreement.
Another reason is that (iii) available spectrum in finite. If a single device could work with all four carriers on the same shared spectrum, then they would need to compete for your business on price and customer service quality. When each carrier has spectrum independent of the others, they also compete on who owns rights to the best spectrum band, who has the best infrastructure built-out and who has the best devices.
Another reason is that (iii) available spectrum in finite. If a single device could work with all four carriers on the same shared spectrum, then they would need to compete for your business on price and customer service quality. When each carrier has spectrum independent of the others, they also compete on who owns rights to the best spectrum band, who has the best infrastructure built-out and who has the best devices.
You can look to S. Korea to see what happens when all carriers share the same spectrum: http://gigaom.com/broadband/mobile-internet-cost-around-the-...