because Google doesn't give a crap about what manufacturers do as long as they keep installing Android on as many phones as possible and in return they get more advertising dollars.
Microsoft has to address the amount of crapware vendors ship and the permissions they have and deffinitely plays some games of chicken with them to try to keep windows market share.
While it makes sense to hit the vendors directly to the extent possible, it also costs these platforms trust when most of the ways users end up with them have them compromised from day one. I.e. do I give relatives a list of vendors I think might be safe to use without a complete wipe and fresh install? For windows that is impossible.
