There isn't anything stopping this. It's just that no one is working on an app. And there isn't any API implemented (yet) for an app to hook in to and fetch posts and comments. Both could be programmed. Someone could also copy the Lemmy API and use arbitrary Lemmy apps with Piefed. I think the developer is open to any of that and I'm pretty sure I read some feature request. It's just that the focus currently is on other things. And Piefed works well as an progressive web app. You can open it in your browser and click "Add to home screen" and you'll get an icon and a browser window that pretty much feels like an app. I'm using that and also don't see any benefit in putting in the effort to maintain an app, when it works well as is.