[mercer]

The crucial difference, as others have pointed out, is that if you start with a dynamic site and add caching, there are a lot of likely gotchas like infinite url's that all load a slightly different page (pagination, tag combinations, etc.), as well as potential security issues when a page is not cached yet, or the cache is busted.

On the other hand, if you start with a SSG, your initial state is that every page is cached and works, and you have to think carefully of adding some kind of pagination mechanism.

As the quote goes, "There are only two hard problems in Computer Science: cache invalidation and naming things." The big benefit of a SSG is that you 'solved' the cache problem, and will immediately be aware of issues there.