That makes about as much sense as "not using server-side scripting."

Personally, I have had great experiences building purely client-side rendered apps, with DB and other APIs exposed to the client via REST. I now consider HTML generation on the server (with separate rendering logic from the client) to be an antipattern -- done right, the same rendering logic should be usable on the client and server, and can prevent whole classes of bugs and frustration.

You make me very sad.

You don't have a choice when, let's say, you're building a mobile app for a service whose back-end you don't control.

Of course you have a choice. You can build a new service and place it in front of the other service. For example using Node.js. But I agree that a SPA is often the right way to go.