Earlier quoted context omitted.
It's because the app has access to and control over the DOM of the UIWebView. Suppose some app called EvilGameFoo is asking you to authenticate with your Google account. They should kick you to a UI controlled by Google, which EvilGameFoo cannot in any way inspect or access, where you enter your credentials. Google then tells EvilGameFoo that they can vouch for you. Instead, UIWebView lets the app asking you to sign…
How are you supposed to know that it is actually a UI controlled by google and not a simulation of a UI controlled by google? The OAuth model not only enables phishing directly even worse than that, it disarms people's natural skepticism towards phishing attempts. Its adoption was a terrible idea.
I have a certificate signed by Symantec guaranteeing that it's authentic!