There are no “truly static” routes in Next.js. Even those static files are revalidateable by Next.js so we have to let Next.js handle those assets and not serve those directly as static files. We have no means to know if a route would be revalidated in the future or not, and since Next.js allows revalidating static files, we have to follow that standard and allow revalidating static files on Netlify as well.
If you want a truly static outputI believe you’d have to use: Deploying: Static Exports | Next.js (nextjs.org)