2) Websites do that through Cookies and Local Storage. These have limits and users would be purging them en mass. The data of the app doesn't disappear for no reason.

3) As I said, the problem is that it runs within a browser if not added to the homescreen. It is a window within a window.

4) Professional or amateur design, websites data is managed by the browser and not you. Caches get invalidated, you download everything again. It happens all the time.

Just being able to do something is not enough, Apps are much smoother experience.