Because it's replacing one closed OS for another closed OS. AOSP is open source and unlike stock Android it doesn't have a bunch of preinstalled spying apps from Google/Samsung/you-name-it which you cannot easily uninstall.
A common user can not be expected to go through with flashing an Android device. A technical user, sure.
But a technical user could also simply go and disable much of the offending apps via `adb` effectively removing them (without uninstalling them) from the device. This is a much less drastic move than flashing a device.
And contrary to belief, Google is not monitoring everything that happens on the device. Let's give the aluminum hat a bit of a rest here.
Plus Nokia is no more: https://www.gizchina.com/2024/02/01/mobile-phone-brand-trans...