They (Verizon, Sprint) already have, but they still use CDMA in outlying areas and for voice service. (Verizon, especially, is known for having the best rural service, at least in the rural areas near where I live.) Verizon does use SIM cards for LTE; I'm not sure about Spring but I'm guessing they do as well.
Both still have the CDMA-carrier mindset of wanting only "their" devices on their networks. In the past they would not (usually?) activate a phone not sold by them even if could work, but I feel like I've heard of people using unlocked devices with Verizon, but it could just be iPhones.