Which is the reason why I choose iOS: I trust Apple to implement the integrations in the WebView for best battery life. To get new OS features in the browser right away. Would Google Chrome implement picture-in-picture on the iPhone if they were running their own engine?
I use Firefox on iOS for the password and tab sync integration, and am supper happy that it’s using the WebKit framework. I care about the iOS experience more than some video codec support.
If the other browsers could implement their engines, you would still have the option not to install them.
So it’s okay for user who (in theory) installed a 3rd party browser on iOS to have a worse experience and potentially make themselves vulnerable to malware? That’s exactly what Apple doesn’t want.
It's just that they don't allow JIT code execution for security reasons which is needed for a comparatively performant engine.