There is an open source version of Android released by Google (AOSP), but it's mostly permissively licensed (pretty much everything except the kernel), and the versions of Android that ship with nearly all devices (even Pixel devices) have modifications that are not open source. It's not just the app store, it's the actual services exposed as APIs. Thus, projects like Lineage and Graphene not being subject, but Google and Samsung still are.

The reason Steam OS is a gray area is the question of what constitutes "the OS". For Android, you can't run apps without the components in question, so it's pretty cut and dry. Steam OS, you can technically use it without Steam: switch it to desktop mode, disable Steam, and run it as just an Arch variant; you can even launch games installed via Steam if you really want. But in practice nobody does that, because you would just use another distro if that was your goal. So, is Steam part of "the OS", since it's the main thing you see when you boot by default, and probably the reason you use that distro? Or is it just a prominently featured application?

edit to add: Here's a graphic depicting the general shape of Android. Pretty much everything except the kernel can have closed source modifications (and even the kernel will typically have proprietary drivers and firmware blobs): https://upload.wikimedia.org/wikipedia/commons/3/32/AOSP_And...