Keeping vertical scroll position stable while asynchronously loading lots of randomly-sized content is harder than it seems.
Yeah, but there's always the option of showing a "fallback" when loading and inserting content in the middle of a feed - one link at the top "go to last bottom of top" and one at the bottom "go to last top of bottom".
A complicated feature I am 100% certain no user ever asked for.
Actually twitter does it better than anyone else and it's the reason I use it over other apps. The User Experience is unparalleled