Thanks for the distinctions and links to the other good discussions you've started!
For the invasive bits that are included, it’s easy enough for GrapheneOS to look over the incremental updates in Android and remove the bits that they don’t like.
That's my approximate take as well, but it wasn't quite what I was getting at.
What I meant is, to ask ourselves why is that the case? A LOT of it is because google wills it to be so.
Not only in terms of keeping it open, but also in terms of making it easy or difficult - it's almost entirely up to google how easy or hard it's going to be. Right now we're all reasonably assuming they have no current serious incentives to change their mind. After all, why would they? The miniscule % of users who go to the effort of installing privacy enhanced versions of chromium (or android based os), are a tiny drop in the ocean compared to the vast majority of users running vanilla and probably never even heard of privacy enhanced versions.
edit: it just occurred to me you may not be a native english speaker, in which case i apologise. "typically not" means it usually doesn't happen.
For anyone who's wondering (from the GIMP manual)