vit microG uses Google services and apps depending on Google Play still use the Google Play SDK and Google Play libraries. microG replaces the middleman (Google Play services) between the Google Play libraries and many of the Google services they use. microG downloads and runs Google binaries for certain components. It's not really a reimplementation of Google services.
microG requires privileged functionality for many of the basics. It needs to be able to impersonate another app for certain checks, which is privileged functionality. Those checks are likely to be expanded until the point that it doesn't work at all without that spoofing.
The whole point of the sandboxed Google Play approach is that it runs as regular apps in exactly the same app sandbox used for the apps depending on it where the Google Play SDK and Google Play libraries run. It provides absolutely no special access. This means it provides no additional access or capabilities. The Google Play libraries could do everything that it can do without it. In many cases, Google's libraries including the Google Ads SDK do work without Google Play. There is no inherent need for Google Play to use Google services. You can see for yourself that Google Maps works fine without it, although it depends on it for some functionality even though that could also work without it. Everything that sandboxed Google Play can do could simply be done by the Google libraries without it though. Google Play could also simply officially support running as a sandboxed app and we wouldn't need our compatibility layer. Our compatibility layer makes it work as simply a regular app, but they could do that themselves if they wanted to make it work on devices without it integrated.
Sandboxed Google Play compatibility layer does reimplement Google Play functionality itself including the Google Play location API, which is provided via a reimplementation using the OS location API by default. We're entirely capable of reimplementing more of the APIs when it makes sense.