We would love to add buildings from Google Maps.
Google did a great job and has excellent buildings.
But, unfortunately, this is a private (non public) technology, about which there is no information.
So we just can not do it.

With Google maps as a provider, some landmark buildings have bespoke 3D representations, i.e. such as the London Eye and London Parliament, which get generated at the right zoom level. In Online Maps the buildings are just a flat extrusion of the blueprint even for landmark buildings and where the provider is Google maps. It would be cool to extend support for such specific 3d representations for the buildings, but again it depends on how difficult this is to add support for and whether such 3d is accessible.