Just because my display is 1440 pixels wide doesn't mean that I want my IDE to take up all of it! The average line of code is much less than 80 characters wide, so if you want to increase the width to 120 characters, you have to increase the width of your IDE, and that means that you have even more wasted display space (all those lines that are only 20-40 characters wide).
Personally, I keep an 80-character terminal, wrap the majority of my code after column 72, and that frees up the rest of my display for a web browser, where I can read documentation, search for algorithms, check my prototypes, etc. I also like to have other terminals open where I read manpages, SSH to other boxes for various reasons, run small code snippets in an ipython shell, perform operations on my filesystem, interact with git, etc. That's not to mention having an IRC/IM client open, a music player, and more.
Sorry if this reply seems to be dragging on, but I really want to point out that when I'm programming, I'm doing a lot more than living in my IDE, so using my full display because some people want to have 120-character-wide lines is really annoying.