Most of the text out there is just not good quality that deserves or rewards deep reading.
My pet theory is that the rampant skimming or "shallow reading" is basically the brain performing a hidden Bayesian probability that any random text put in front of us isn't worth the effort of deep reading.[2] This is why many of us go to HN comments first instead of reading the actual article. The Bayesian priors told us that the "tldr" in the comments got to the point where the article all-too-often had a self-indulgent author that meandered all over the place and wasted our time. Therefore, "shallow reading" isn't bad for us... it's our way of optimizing against "information overload".
Even college professors who are used to heavy reading workloads skim new work. I'd argue this is another manifestation of Bayesian priors.
To back to my C Language example. I didn't really learn C by reading those binders cover to cover. (Deep reading.) I really learned it when I did shallow reading across fragmented sources like USENET comp.lang.c forum and playing around with toy programs. So maybe deep reading isn't the answer but the attention restriction from not being distracted with Twitter and Instagram notifications. In other words, maybe we're conflating benefits "uninterrupted study" with "deep reading".