We touch on this in the docs [0]. TLDR, bundlers are still necessary (for now) to reduce the number of roundtrip requests required before page load (ideally to 1), and in more complex apps to avoid overwhelming the browser with parallel requests. Even HTTP2 can't handle 100k parallel requests for a big app with a bloated node_modules.

[0] https://bun.sh/docs/cli/build#why-bundle