The OS is but the Google Play backend isn't. Google has a monopoly over Android by keeping a monopoly on the appstore which dictates that you must allow google spyware to run on your OEM fork to be able to qualify as a "Secure Device".

Several Chinese OEMs have China only variants that don't use GPlay and also ship with some some other cool apps, but they can't sell it globally because Google says "screw you" since no one publishes apps outside Gplay, and because several major apps refuse to run on Googless android which GrapheneOS has threatened to sue

This is still just the tip of the iceberg though. Google already got sued for GPlay monoply last year and reached a $700 settlement just for developers.

On top of that, several of Android's underlying features are considered archaic and dated. They always have huge kernel patching issues because no OEM (especially Qualcomm) releases the source code for proprietary binaries, meaning no one easily upgrade kernels (practically impossible for FOSS android, expensive for OEMs). The android runtime is imo a piece of crap compared to some low power optimized linux distros. ADB is still needed to delete system apps. Settings lies about permissions, which themselves are poorly sorted. Oh and Google hired the dev behind Android rooting (magisk) so they could kill magisk hide which circumvents system app abilities to tell if you are rooted and therefore not worthy of running proprietary apps.

There's so much more the deeper you go, it's just really hard for any contender to step up because of the sheer might Google has over the market. They have so much power that they coerced Samsung into dropping RCS support which makes Google Messages the only app on android that supports RCS, even though RCS is an open OEM standard from 2008