With LiveView, you've got one code base. There's no frontend/backend split, so you don't think about the API, you don't worry about going back and forth between JSON and all the associated error handling, you don't worry about versioning and keeping deployments in sync when/if you break APIs.

You shouldn't really compare it to React, you should compare it to your whole stack, and see how a lot of the code and the worries just kind of go away.

This is much like how it used to be with classic server-side rendered Rails/PHP/Phoenix... except with LiveView you can update part of the page running on the client, in response to events coming from the client without a page refresh.

Eliminating a bunch of API code.