A company like Vercel needs significant infrastructure and the people associated with it, as well as supporting admin (HR, lawyers, accountants), proper customer support (for 24/7 coverage so that can be quite a lot of people), UI/UX people, documentation/education people, etc. etc. Of course also sales and account managers, presales engineers.
Just because you think you can make their product in a weekend all on your own doesn't mean that to get to where they are a ton of people aren't needed.
I agree it’s odd, and I’m not sure, but it feels to me like a big chunk of their staff might be involved with building Next.js and also running the annual conference of the same name which gets a lot of attention. I think maybe they see this as their moat. Any product that is just a thin layer around AWS is in danger of being suddenly eaten by a competitor or an OSS alternative or AWS itself. For Vercel, owning one of the world’s most popular web frameworks, which just happens to be particularly optimised for their own platform (both the deployment DX and performance), probably gives them a decent moat in that it keeps a huge number of devs around the world familiar with and positive about Vercel.
Just because you think you can make their product in a weekend all on your own doesn't mean that to get to where they are a ton of people aren't needed.