Partly because I work in video games, so at work I benefit from having one landscape monitor for testing the game at a realistic resolution and/or using the tools, almost all of which are always designed for use in landscape orientation.

(I could probably work around all of that, and indeed when I work at home I just suck it up and/or have things stretch across two screens. But the other part is that the monitor stand I have at work is a bit limited, so if I have both monitors in portrait orientation and move them so they're adjacent, they end up too close to me.)