> Can't we just go back to node + npm + vitest + vite?

You're kind of stating the reason for the buzz there: one runtime that does everything you need rather than cobbling together a dozen different tools.

Not that there's a problem cobbling together different tools made to do a job, each tool has a purpose and solves a problem, but having all problems solved out of the box is very convenient.

Bun does offer an additional major advantage, i.e. the bundler has a runtime API, so the same process that serves your assets can also bundle them in memory without having to coordinate an external bundler writing static files to disk.