It's the carriers that want the phones locked down so that they can have their bloatware installed and irremovable, Verizon in particular is the most notorious

It's the carriers that want the phones locked down so that they can have their bloatware installed and irremovable, Verizon in particular is the most notorious for mandating this. Additionally, among manufactures, Samsung wants to appeal to enterprise and gov environments and so also has their own suite of security features branded as KNOX and typically locked the bootloaders of the American/Snapdragon models. The good news is that you can buy almost any Android phone off Amazon and with the exception of Samsung it is most likely bootloader unlockable. Motorola is especially good in this regard.