It's this. If you look at the stats on the % of visitors to most web sites, 1280x1024 or 1024x768 (and the non-retina Macbook equivalent) remain the dominant approximate screen sizes, so developers optimize for them on the assumption that things will work ok for folks with bigger screens.
I can remember when we used to optimize for 800x600.
I can remember when we used to optimize for 800x600.