How many people are using the camera in pinned web apps? What's the app you use? I'd imagine most camera-related functions are already best served by native apps.
It could also be a security/privacy decision to leave it broken but safe until they can implement camera access through WebViews securely.
The closest to any official reason I could find is a dev letting us know that mum's the word:
>I asked about this internally and the answer is that, right now, WebRTC is only supported in Safari. No WKWebView, not even SFSafariViewController.