In the Next.js/Vercel world, this hybrid architecture is primarily (I believe) an issue of cost savings (for them and you). For $20/mo Vercel will host your hybrid Next.js side, yet do all of the backend configuration and maintenance on your behalf, configuring not just NPM but Lambda and Cloudflare Workers, along with nginx and caching and CDN mirroring and invalidations, all seamlessly.

So as a dev you can just code against the Next.js docs and not have to play devops. That's a LOT of time savings for $20/mo, and you can afford way more page hits this way than a barebones $20/mo NPM VM would get you. Yes, backend it's super complex and fanned out to different providers, but Vercel manages all of that for you.

I imagine Gatsby and Netlify are similar, but not sure. Next.js was designed around Vercel specifically (sadly) and there's a lot of vendor-specific lock-in features.