Your hacker biases are showing.
The sites where 95% of people spend most of their time (Twitter, Facebook, Instagram, etc.) are all built as single page "applications." Users expect rich interactivity from websites these days and are usually not tolerant of UI refreshes.
The comparison is not to "desktop" applications but to mobile apps. Users expect a mobile app level of interaction. Plus, if you're building mobile apps then it's a lot easier to just reuse those same APIs for a web app.
That being said, if your site doesn't have any interactivity then it does make sense to just render it statically.