There's not enough emphasis put the fact that designing for one aspect ratio/resolution is much easier on the designer. Not having to debug scaling issues or 1-2px vertical borders saves metric boatloads of development time.
There are two main reasons fixed-width layouts are still so popular on the web, even though responsive layouts are well-supported. One, fixed width maps 1:1 to Photoshop. Two, it's simple.
And three, it feels icky to have to add "margin-left: 3.448374%" to responsive stylesheets because CSS really wasn't built to support complex, percentage-based layouts.