JAMstack sites are often seen as being static. A more accurate mental model for them would be that they are sites which have the ability to be hosted statically. The difference might seem semantic, but thanks to the rise of many tools and services which simplify running a build and deploying to static hosting infrastructure, such sites can feel much fresher and dynamic than you might imagine, while still capable of being served from static hosting infrastructure, with all the benefits that brings.
A feature often used as an example of why a site cannot be hosted statically is comments. A comments engine needs to handle submissions, allow for moderation, and is by its very nature, "dynamic".