Cost is also often a non-issue. Let's say a desktop can save you $500 or so, or even $1000. That's at most $2.5 a day if you keep it for two years, it just isn't relevant to a developer making 50, 100 or 200 k a year.
When it comes to screens and ergonomics, that's largely subjective. For some tasks, I do prefer a large screen, for others it doesn't matter. But I always prefer a macbook keyboard with the built-in trackpad, super convenient.