In 2020, the majority of websites are a bloated mess. I should know; in my day job at Silktide we build automated software that tests millions of websites every year. The average Lighthouse performance score for websites flowing through our platform is just 38 out of 100. And before you ask, these are not sophisticated websites - they're mostly 5-10 page brochure sites for SMBs.
In almost every case, these sites use unnecessarily bloated frameworks to build their pages on-the-fly. Whether it's server-side like Wordpress or client-side like React, there's a bunch of stuff that happens every single time someone views a page - and that stuff takes time, making websites slower to load.
Static site generators do away with all this nonsense and as a result they're lightning fast. That's one of the key reasons this site scores 100% for performance in Google's Lighthouse audit.
While most websites are put together in real time as the visitor is looking at them, static sites get compiled to the basic building blocks of the web in advance.
A static site generator allows you to write your content in a friendly format like Markdown. When you're ready to go, the generator combines your content with snazzy templates to generate static HTML and CSS that can be served to visitors.
While static sites are super fast, they're not the ideal solution for everyone.
As a developer, building a static site is second nature to me. However, it would be an intimidating experience for the majority of people. There are headless CMS solutions, but I'm not aware of any ready-to-go hosted services like Wix or Squarespace.
Additionally, it seems blindingly obvious to say, but static sites are just that: static. Highly dynamic websites aren't going to be well served by a static site generator (even though there are some ways to add interactivity like Netlify forms).