It may be evident for you since you probably have seen many frameworks and implementations. Before hearing about Jekyll, I only used Blogger and WordPress, which relies on the database for almost everything.
I guess what I'm saying is that (except for the comments) blogs are static pages that contain (mostly) unchanging content, not dynamic data that needs to be updated.
If you're curious about the history, you might look into MovableType, which predates WordPress by a couple years, and has always supported static page generation: https://en.wikipedia.org/wiki/Movable_Type
http://dennisbareis.com/ppwizard.htm
Nowadays I am using Hakyll, it's a natural progression from Jekyll and is much easier to customize (it's basically a Haskell DSL for static site generation).