The problem is that Google seems to be only one writing good software. If you have used Tizens of mobile vendors or customised Androids they tend to be crap. Google keeps software quality up, forces vendors to roll out patches.
Android itself is open source. The issue is Google play services that provides a lot of functionally that many apps rely on. You can run something like lineage os or graphene os, it works great for basic phone and browser functionality.