Because app developers will use other browser engines allowing them to ignore any of the privacy constraints in place at the moment.
Also an app can access and send more data than a browser window. Anything the webview can block can just get transmitted by the app itself separately. So there's no reason to block 3rd-party browser engines as the GP post says.
The real security issue is: if a bug in Safari is found (it happens quite often) users cannot use another browser in the meantime.
Not for most people.
That still does not force you to install those browser engines.
If you want the apple approved apps, in the apple approve app store, that fine. But me using different settings does not force you to not use those approved apps.