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.
Of course, you're absolutely right that it would be trivial to spoof, say, Google or Facebook or Twitter and collect credentials. I would _hope_ that behavior would be detected by Apple or Google during app security screening, but maybe not. Regardless, I totally agree with you about OAuth and security. It does present a lot of problems.
I have a certificate signed by Symantec guaranteeing that it's authentic!
Outside of the Android model where it does the auth for you upon request and only hands the app a token, I don't really see how you don't open yourself to this attack.
iOS really doesn't provide a way for other vendors in an inner ring of consumer trust and power to deal with this. While I'm not a fan of the decision to give full access to Pokemon Go, I didn't find it surprising that it did so on iOS and I had to make a conscious decision to "trust" Pokemon Go as part of the process.
If UIWebView didn't allow the app access to the DOM then it wouldn't be possible this way. But,of course, the user has no way to know how the UI is implemented.