Isn't the browser also an extremely fast Javscript bundler? How many scripts does a site need to make it feel faster when bundled? When I visit websites that are rendered serverside, they usually feel instant to me. Even when they load a dozen scripts or so.
What ESBuild offers over plain ESM is: - fast TypeScript/JSX compile - bundling shared code to reduce request waterfall/splitting to reduce redundancy - bundle optimization (tree shaking/dead code elimination; minification is actually faster than not using it) - a simple plugin system for use cases like other compiled frameworks like Vue or Svelte, or whatever else you might want in a build pipeline
While a website that loads scripts per page will only load the scripts that are needed on the current page.