Because they have these things called SDKs to extend their product, for specific programing languages.

Here is an example, https://www.sanity.io/studio

Want to extend the SaaS product for your backoffice users?

Learn React or make Studio from scratch.

Frontend stacks are also backend stacks in cloud products.

This looks like a SaaS vendor not a cloud provider?

I wrote "SaaS cloud providers", I am yet to see a Saas solution on-prem.

Because with Next.js, Vercel was able to turn the frontend stack into also a really shitty backend stack. And it's particularly shitty at being deployed, so they're in the business of doing that for you.