nostr:npub1yaul8k059377u9lsu67de7y637w4jtgeuwcmh5n7788l6xnlnrgs3tvjmf can probably speak more to why it hasn't been done yet. He's working on one for iOS called Yeti, and from conversations we've had in the past, certain things about how iOS works make it a more difficult task than on Android.
Discussion
It has nothing to do with decentralization. Apple sandboxes its apps and limits background and cross-app activity, presumably for security and performance reasons.