In the US specifically, it also doesn't help that:
* that virtually all new phones are sold carrier-locked
* most of those through a carrier subsidy
* the secondary market for used phones is far less well developed than in other countries
* until quite recently, something like half or a little more of all new phones were non-SIM-based CDMA phones (and that number is still non-trivial)
...means that the carriers have a great deal of power over the phones offered.

The one major exception used to be "world" phones from the CDMA carriers (mostly Sprint and Verizon) where they would have an unlocked SIM slot for the GSM or GSM/UMTS radio (or in a few cases, a locked slot that allowed anything EXCEPT the major North American GSM carriers.) That was before LTE ... now the phones need a carrier SIM card for LTE, or have a hard-SIM and no SIM slot.