Maybe your single page app is different, but the ones that I know break most of my browser’s features, such as the back and forward buttons, page refresh, bookmarking, sending a link, or opening a link in a new window or tab. They offer no way to link to something that I look at. (Oh, I know there are exceptions to this rule, but they typically require effort — a lot more than many developers are prepared to invest). They are bloated and slow to load, even though the actual information they display and the interaction they offer is very simple.
Every time you extract a collaboration between objects to a collaboration between systems, you’re accepting a world of hurt with a myriad of liabilities and failure states. What to do when services are down, how to migrate in concert, and all the pain of running many services in the first place.