Google in most cases is not the device's manufacturer, and in a mobile device OS and application SW are tightly coupled, so you cannot really have OS updates separate from firmware updates, bypassing the actual manufacturer's own updates.
Google (and Apple and Microsoft) can totally do it for devices that manufactures and maintains on its own, and actually it is one of the selling points of their new phone.
In addition, at least in the US, devices connecting to a cellular network have to go through a certification process, enforced by the carriers, and every firmware update requires additional regression testing (time and money) for the device to be admitted on the network again.
(That's my guess of why Lenovo dropped the policy of fast updates for Moto phones when they bought Motorola)
Google (and Apple and Microsoft) can totally do it for devices that manufactures and maintains on its own, and actually it is one of the selling points of their new phone.
In addition, at least in the US, devices connecting to a cellular network have to go through a certification process, enforced by the carriers, and every firmware update requires additional regression testing (time and money) for the device to be admitted on the network again.
(That's my guess of why Lenovo dropped the policy of fast updates for Moto phones when they bought Motorola)