This is partly done, I think, to prevent users from uninstalling bloatware (Chinese brands doing this mostly), since I've had to deal with this on a BBK branded phone, locked down so far even ADB can't touch the bloat.

Google is also a part of this with play integrity and apps being blocked from working, so if you depend on Google or if you want to use that phone in your day-to-day life with apps from work or banking apps, it might not work great on the same phone.

In my opinion I think it is mostly the manufacturers fighting this, Google is solvable but a locked bootloader isn't.