Yup, in the world of soundproofing, windows can be (and are) considered a major problem. The misconception occurs when those who are uneducated run out and try to solve their soundproofing problems by replacing all of their windows.
Yes, windows are a major source of noise. No, replacing them with “soundproof” glass doesn’t usually work. In the end, adding a second pane of glass over the one that already exists, creating a 1-inch gap in between, is more effective.
Sadly, you can’t use Green Glue on the windows themselves. Otherwise, we’d be creating our own windows!