Apps to Web.png
It's 2013, we have a prevalence of widescreen aspect ration monitors, yet we have a web built for last decade's resolutions. What gives devs? If we can have apps that scale, why don't we have websites that scale? It's not rocket science to create something that would scale to fill in the empty voids.
Using Twitter as an example, against an app with different content and on a minority screen resolution...
Websites do change for resolution and can do so far better than apps usually do. Apps usually just "scale" (websites can too), which is understandable for phones and tablets with little size variation, but sub-optimal at best for desktop/laptop computers. But it all comes down to a website's goals and target audience. In the case of "web apps" such as Twitter, Facebook, et al the website is usually aimed at desktops with apps for devices. Your everyday websites will usually "respond". Then there are the ones that sit between and have a dedicated mobile site (Neowin does this afaik). It depends on a website's goals as to which you choose. In Twitter's case there is no need to "respond" as they have apps.
The only benefit to Twitter being responsive would be for large screens. But despite what you assume, you are viewing the world through an uncommon screen resolution (presumably 1920x1080). The large majority of users have a horizontal resolution of 1600 or less, with most of those at 1366 or less (source). Twitter could go to the effort of filling the space, but there are only two less-than-ideal methods available. First is to just make the page wider, which would look damn ugly (one tweet per line) and be bad for readability (narrow columns are easier to read). Second would be by adding "filler columns" with content from others page like MetroTwit does. However, that would give users with large screen resolutions different content, which is pretty much a no-no on the web and not what responsive design is about anyway.
Although responsive design is quickly gaining traction, you're rarely going to see a great degree of responsive design on "web apps" though. It is too complicated to do (technically and visually), large resolutions are an altogether small market, desktops are a decreasing market for "web apps" and devices have dedicated apps. You'd need some pretty solid reasons to go to the effort. So "web apps" usually either have a dedicated app and/or serve a mobile-specific page.
And web development is like rocket science (or at least feels like it). Guess who/what we have to blame for that.