Because it's better, and I like more. For mobiles and tablets you give 100% of the space which is fine, for desktops you give an extra small surrounded by two rivers of dark grey.
Why don't let use the space available? But the real answer to your question is: why not?
You can get more sensible use of screen space if you have the forum in a window that is sized just below the limit for getting a sidebar. The line length is significantly better and there is no wasted space.
Why??
Because it's better, and I like more. For mobiles and tablets you give 100% of the space which is fine, for desktops you give an extra small surrounded by two rivers of dark grey.
Why don't let use the space available? But the real answer to your question is: why not?