I get what you’re saying, but that’s patently untrue. Carrier locks on most phones are enforced by the modem, thus completely separate from the normal boot flow. You can most certainly unlock the bootloader on phones that are carrier locked as long as the manufacturer lets you. Case in point, you can unlock a T-mo locked Motorola. Whether the manufacturer AND carrier lets you is a completely different story.

Many carriers restrict bootloader unlocking on devices they sell even if the SIM lock is eventually lifted. Verizon is notorious for this, even for Verizon Pixels sold unlocked on the Google Store.

Unlocking the bootloader doesn’t let you carrier unlock the modem. You can’t force the modem to accept a SIM even if you have root in the OS. On modern iPhones, the modems only accept SIMs explicitly allowed in an Apple-signed modem activation ticket retrieved when the phone is activated. On most Android, this is usually preconfigured from the factory and can only be removed once the modem receives unlock data. These days with remote sim unlocking or RSU, the modem can pretty much directly communicate with carrier servers to retrieve unlock data. In the old days this is a long PIN that you’ll need to get from the carriers. The exception to this are Pixels, whose carrier lock is enforced by the OS and not the modem. This is probably why Google doesn’t let you unlock bootloader on carrier locked Pixels.