Because app navigation is not linear like the url history. Think of a popup with tabs within a page. There's the navigation within the popup, and the navigation within the page. When you close the popup, you don't want 'back' to bring the popup back, you want it to go to the page before the popup. This is hard to replicate with just urls and server side rendered html.

Also you don't want that store server side because there can be multiple parallel tabs and you don't get notified server side when the tab is closed to properly cleanup the associated resources.