The browser launched by the app is owned by Android device. It actually uses the native browser engine in order to render the website inside the app. In addition, the url is shown for security purposes so users can confirm that they are really at SteemConnect.
While it is possible to open the browser in an extra app, it will be less intuitive and will add more steps to the UI. In addition, is nearly not possible to have a callback from the native browser application. The way that SteemConnect works is by using callbacks. In our case, our callback will redirecgt back to a dummy localhost address which is used to capture the accesss token and close the browser renderer at the app.
While for websites it is suitable to open another tab with SteemConnect because you can use the callback, it is impossible on phones since the callback cannot redirect you to the app.
Also, all the source code is there for audition. The plugin used to launch the browser in the app is itself a component of each Android device.