According to https://blogs.windows.com/windows-insider/2021/10/20/introdu... the Microsoft based support for Android apps is powered by Amazon's app store. Even if the same games are available under the Amazon and Google stores the games typically save progress in a store specific way.

If you want to "Seamless sync across devices" as mentioned, you need to use the same store as your primary mobile device.

This is being done because of that.

Google does not want to lose the PC market. Amazon will get it, if there is no Google alternative.

Google does not certify WSA builds for GMS, so you won't get Google Play Games integration, which is one of the key features touted by Google in this announcement.