That’s a good question!
The Google Maps SDKs have their own custom rendering pipelines which are platform specific, and is also doing 3D rendering(e.g StreetView). Flutter at the lowest graphics layer is using Skia which is a 2D graphics library. Right now there are no GL/Metal/Vulkan bindings for Flutter apps which will need to be there before a “pure Flutter” Google Maps implementation can be done. Additionally, as you mentioned, re-implementing Google Maps would take a very significant effort.
It will be awesome if one day we get a “pure Flutter” Google Maps implementation, the one thing that’s important to be doing today is to make sure the public API for the Google Maps plugin is not becoming platform specific, so if/when the day comes we can switch the maps backend as seamlessly as possible.
[Disclaimer: I’m on the Flutter team and has been working on the Google Maps plugin and the platform views support]