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 usually keep my resolution low. 1366x768 is fine for everything I do -- including movies, gaming, and development.
The main reason I do this is to make the default font sizes for programs, websites, and OS's more readable without visual glitches or breaking layouts, as zooming has an unfortunate tendency to do.
When viewed on the screens most people here are using, they're unreadable.
I find that 200% zoom works for Full HD screens (and is acceptable, if a little big, at 1680x1050)