Maybe it's fairer to say that it's too long to read for what they feel they'd get out of it. I think that's an important distinction. There are some people who are what you're complaining about, no doubt about it, but there are also people who feel that the opinion of a relatively random person on the Internet is only worth so much of their time and effort. Doubly so when it comes to a point-by-point debate of that opinion.

EDIT: But it's true that there's no real call to take the time to point it out, unless they actually do want to read your opinion but wish it was in a bite-sizier package, and don't know any faster way to tell you so.

It is also fair to say that reading text on a monitor is harder on the eyes than reading text on a page, though how much that contributes to the Internet's relative brevity I can't guess.

TL;DR summary: shit be exhausting, yo.