They don't have to be static but making them dynamic might not worth the cost.

From a simple static page, now you need an API service, most probably connected to a DB or somehow integrated to the rest of your backend. So markdown suddenly isn't enough and you need some server-side logic.

For random strings, you can do this with client-side logic, which in some cases might be easier than server-side logic. But you are still moving from no-logic (static) to somewhere-logic.