Because that is all the Google apps are really, are one iteration of a group of apps for functionality. You can always use F-Droid and install great open source alternatives.

The fact that you can install a different operating system on some android phones doesn’t mean that the Android that phone manufacturers actually use is open source.

Indeed the fact that you have to install a different operating system that isn’t even called “Android” proves that Android as people know it is not open source.