As I've said, the basic reason it's not desirable is readability. Majority of the web has gone fixed width for a reason, despite screens becoming higher density and wider. Long horizontal lines of text have been proven to reduce information retention, scanning efficiency, and other factors that basically reduce your ability to find and absorb relevant information.
In addition, wider width complicates the design. It's not like we have a staff of web designers waiting for their next project to accommodate how to best address content and style goals into > 1920px.
But my final point has to do with the last screenshot you posted. Your eyes will be slower reading the 100% width screen of the forums vs a fixed width condensed version. In the 100% width, your eyes will catch the left column of topics, and for each row your brain will traverse across empty whitespace to see when the latest post happened and who posted it. Your eyes will not jump to the right column, even though you might think so. They have to go horizontal across the empty whitespace because they need to trace the horizontal lines to make sure you're in the proper row and reading the relevant information.
Fixed width / more condensed eliminates most of the center whitespace traversal - sure, a lot of unused pixels around the content, but a) at least there isn't a lot of unused pixels within the content, and b) unused pixels around the content frame it so your eyes/brain know where to focus. We don't want you looking in the gutters anyway unless we put something there (a very real possibility at some point in the future for important notices). It really is that simple.
Folks, this layout was not just thrown together. A fair amount of research and experimentation went into it with the goal to make the website a better tool for you. Are we more interested in pixel efficiency or brain efficiency? The response to this layout seems to imply pixels are more important than brains.