I was curious as well. I think this answers the question:

https://vitejs.dev/guide/why.html#why-not-bundle-with-esbuil...

> While esbuild is blazing fast and is already a very capable bundler for libraries, some of the important features needed for bundling applications are still work in progress - in particular code-splitting and CSS handling. For the time being, Rollup is more mature and flexible in these regards. That said, we won't rule out the possibility of using esbuild for production build when it stabilizes these features in the future.