Because both are opinionated ways to get a React project going without having to configure any tooling yourself. Next.js also focuses on client side JavaScript

Because both are opinionated ways to get a React project going without having to configure any tooling yourself. Next.js also focuses on client side JavaScript, but happens to render at the server first, so that there's something there for users before the client bundles have downloaded, parsed and been executed. You could think of it as a more configurable create-react-app that supports server side rendering, with a convention over configuration approach to pages and links.