I think Vercel and Next.js are built by the same group of people, the same people who made Now.sh, started the company (Zeit), then changed product name to Now.sh, then changed company and product name to Vercel.
Not defending NextJS, I'm pretty out on it myself, but ISG requires a server to run. It pregenerates static content for defined pages and serves that until revalidating. If you've built a fully static bundle, nothing exists that would handle that incremental/revalidating logic.
