You can fork AOSP, which I'm sure can't run any of the top 10 apps of the Play Store unless you install Google Services, which are closed source and require a licence from Google (which in turn requires you to bundle Chrome, make Google Search the default, etc).
AOSP by itself is pretty useless because it can't run any of the apps the public is interested in.
*Can't run many of the apps. Lots of apps will run with limited functality. Chrome generally runs on AOSP, but it won't update and isn't nearly as capable as it would be with Google Services installed.
You can't freely fork the Android that's dominating the market: Android with Google services in it, key components of it are proprietary. You can't even sell some devices with it and others with an AOSP fork.