> Why would I want a webpage which stops showing additional content after ~1200 pixels wide to take up the entire of my 2560px wide monitor?

Personally, 80% of my web usage these days is hacker news and wikipedia. Neither of which do this.

The rest is probably majority dev docs (crystal atm), and I'm not aware of any dev docs that do this either.

The point here being that not all websites exhibit this behavior.