There's a lot of features that Chrome on Android supports but that can't be added to Chrome on iOS because of Apple's policies. Important features, like fully offline-capable webapps.
I can see your point, but every platform has its own restrictions. Desktop Chrome can do a lot that Android Chrome can't. There are real security benefits to not allowing downloaded code to run on a device without passing through Apple's JavaScript engine. Of course security and features are often at odds. Outside of tech forums, I've never heard anyone complain about missing features in iOS Chrome, so I'm just saying that to most people, the interface, bookmark sync, etc. is what makes it Chrome, not the JavaScript engine, and the tech it's built on will continue to change over time, bringing new features and new restrictions.
Different HMDs are arguably more similar than different GPUs, and they manage to support the same software, but in the early days there were plenty of Glide-only games. That didn't work out so well. Most people will probably just use tools that optimize for any popular HMD with little enough effort that only contracts will make exclusives make sense.