Well it seems like either each and every website could build offline support and background sync into their site, or you could use a browser that does it for you.
In the former case, if you rely on sites integrating it, you could be frustrated when some sites do not implement background sync and offline support (or do it badly).
Having the browser do it seems simpler. Especially given the real world use cases for this feature.
(I'm not against having these technologies on the web. Just seems more sensible for a browser to do your reading list — just like it manages your tabs.)
If only we had a standard way to do this...