That's mostly because `create-react-app` was abandoned. There is no default project template for React SPA, so everyone just uses Next, even if they don't need SSR.
Add in tailwind and a router of choice (I love wouter) and you've got 90% of what you need to build most applications. Vite is so much simpler than webpack and the rest that came before, it's a tiny amount of work to do this.
A lot of things exist. But to become a go-to standard a good timing is required. Next.js was getting popular right at the time Facebook abandoned 'create-react-app' template, and many people adopted Next.js even if they didn't need SSR. Vite didn't exist back then.
Add in tailwind and a router of choice (I love wouter) and you've got 90% of what you need to build most applications. Vite is so much simpler than webpack and the rest that came before, it's a tiny amount of work to do this.