It's more complex than that.

RHEL has Fedora upstream. There's a group of people who regularly contribute to those projects on their own time and the userland for Fedora is made up primarily of FOSS where people routinely try to consolidate popular features into main code branches. There's a truly free software project that is the main project that someone provides paid support for. Fedora drives the evolution of the system; RHEL just gives a way to make that evolution palatable to the suits.

Android has the AOSP but it's not the driver of Android as a platform. For the most part, the evolution is driven by a bunch of closed-source applications that Google and OEMs happen to run on Android. Those parties derive competitive advantage and brand identity from the proprietary code that runs on top of the Android OS, and don't make a habit of merging much of that into the project. There are the system-wide improvements that get updated, sure, but the ethos is not to keep the code moving up the chain into the project so that anyone can take it and do what they want with it for free.

It's a hard difference to describe but it's there.