IIRC it's actually a technical limitation. Pre-installed apps are installed on the system partition, which is read-only. If they installed them on the user partition, they wouldn't survive a later factory reset. That's why you can remove them if you root the phone.
They could of course fix this limitation, but I guess the incentive isn't there for them to do so.
That's a win win as far as I'm concerned. When I uninstall it, it's gone. If I have to factory reset, I don't have worry about uninstalling it again. I don't know or care why bloat gets preinstalled & can't be removed. I care that there's something on my phone that doesn't need to be there & I don't want it there.
They could of course fix this limitation, but I guess the incentive isn't there for them to do so.