The operating system level does not have as much information as the browser so it would be much less effective. You could do domain-level blocking (mostly) but miss lots of granularity on URLs, types of requests, contexts of request (what's the URL of the parent frame?), etc.

Also it would be impossible to perform cosmetics injections (scriptlets, alterations of the DOM, etc.)

It might already provide a good base-line but definitely not enough for all cases (e.g. YouTube ads, etc.)