Would you say that the use of less and less informative pop up windows is the result of all the really stupid users that exist in the world?

Actually one could argue the exact opposite. Then of course is the issue of nobody reading the popups, but still it could be argued that such miserable state of things has been caused by bad copy in popups...

In my opinion that can be give it to 2 causes. 1st. Popup windows became such a commodity that users who where constantly annoyed by it tend to close it inmediatly, so shorter copy it's more easy to digest in that split-second before they close it, in that scenario the copy on the popups have been changing to a more call-to-action focus.
2nd. It has to be related to the adaptability (aka responsiveness) of the content, less text more easy to adapt and less time for the user to read.

tl:dr
Time and space, time and space...