Because the apps replace an app that is already on the phone that is signed by a different developer.
Nonsense. As if a user wouldn't realise that "Gmail" isn't actually "Gmail" any more. The only thing that you can achieve is that someone opens your app exactly ONCE, because he thinks it's Gmail. However, you can do exactly the same by actually installing the App you initially promoted in the phishing link: "Hey, install Flappy Bird 2", then install an app called "Flappy Bird 2". A user who installs this app will also open it. The ability to install over an existing App, which, by the way, is not even likely to be installed in the first place, brings nothing new to the table for the attacker. This is therefore really a non-issue blown entirely out of proportion.