By this logic - safari/firefox/chrome should not be allowed as an ios app because you can't review the usability of all the games on various websites that run within the browsers.
It's more likely they make we webkit mandatory so they can control the pace of progressive web apps and not have their platform be co-opted by Google through the web.
Personally I use Android partly because of the lack of web engine choice on iOS, but this isn't strictly true AFAIK. Apple does require the same WebKit/JavaScriptCore, but other code beside the UI skin can be different, such as the networking layer, and for Chrome at least it is different.
I understand that you end up with a skinned webkit and JavaScriptCore - A few years ago I wrote bindings for JSC for a mobile game engine so that you could invoke native code from js in an ios app.
That aside, my point still stands that they can not possible vet content on the internet, so why should they be able to apply this to streaming games also.
Perhaps netflix would have been a better example.
Do they vet every piece of content netflix have, if not then they can't reasonably apply this to games.