Why static sites still win
A static site is a folder of files. That simple fact explains most of its advantages.
Speed
There is no database to query and no template to render on each request. The server reads a file and sends it. Add a bit of caching and pages open almost instantly, even on a slow connection.
Reliability
Fewer moving parts means fewer things that break. There is nothing to patch at midnight, no plugin that stops working after an update, and little to attack.
Longevity
Plain HTML written years ago still opens in today's browsers. Few technologies can say the same.
When it is not enough
Sites that need accounts, comments or live data will need a backend. But many projects, such as portfolios, documentation and personal notebooks, never do.