• I have never met a person that just rooted their phone without slapping a custom ROM on it.

    Now you have

    The initial motive for rooting is normally to install a custom ROM

    I think there are some misunderstandings.

    Root is not needed to flash a custom ROM. That does not (usually) happen from a running system, but through fastboot or the recovery, and those don’t ask for root permission, only an unlocked bootloader.