It's an employee benefit because it makes work more enjoyable. Being stuck in a problem that you can normally tackle with relative ease when "in the zone" is extremely frustrating.
Sure, but to the employer the primary benefit of this is that people don't get stuck in problems that they can normally tackle with relative ease; the main effect (to the employer) is the fact that they're much more productive, not that they're a bit more happier.
"Enjoying your job" is a perk most people don't get. To them, larger monitors, better laptops, offices with doors, etc are simply tools people use to feel better than you.
Software engineers get to enjoy their jobs, at least in 2017. Until humanity either produces way more developers or makes us obsolete, we're basically free to enjoy our position of demanding work be enjoyable.
