Do you feel like this is ultimately related to the medium or to the content as well? Intuitively new media won't have as much (good quality) content as books, so that might be the main issue here.
Taking a break from all of that stimulation is a good way to quiet our minds and relearn how to focus.
Another kinda big reason I can find it harder to learn via the internet vs books is because there's so much deception online. You get primed to always think and try to figure out "ok, how much of this is marketing, or signalling, or the author just spewing BS, and how much is something I can rely on", which takes away cognitive resources from what otherwise could have been spent on building a rich and secure knowledge-base.
care to share that intuition? this doesn't seem obvious to me unfortunately