In fact writing an REST/Web API should be easier than writing a server-side generated HTML website (no need for templating language, ugly form frameworks,...).
Why is that easier? It's more work, you are now rendering two views instead of one, a JSON one(server) and HTML one(in the client) with all the JSON encoding/decoding that it entails. You are still using a templating language and, dealing with forms in React is more cumbersome than doing it server-side.
separation of concerns, easier testability, easier mocking..., the thing is especially in more complex applications the code that generates/validates the data and the code that displays them are usually written by two different people.
nowadays once the json schema design is settled, they can work in parallel, each of them can test their parts without needing the other and the merges can be simpler, because the parts do work more or less stand-alone.
Why is that easier? It's more work, you are now rendering two views instead of one, a JSON one(server) and HTML one(in the client) with all the JSON encoding/decoding that it entails. You are still using a templating language and, dealing with forms in React is more cumbersome than doing it server-side.