package.json doesn't have any place to define the styles and templates your component exports, so browserify can't meaningfully resolve dependencies and build your assets for you. For really good support, you'll need to be able to rewrite the URLs inside CSS too due to different directory structures within each component. Component does that for you.

That's a pretty fair point about the CSS url rewriting, it's possibly the kind of thing you can fix with a browserify transform, but iffy and as you say component is doing it already.