While it's true that a laptop is more flexible in the sense that it's easier to pack up and set up, the problem is that few places are really good for work. While I can use a laptop in any room in my home, only the office has a suitable chair. And the only other desk with suitable height and size is the dining table. While being able to work from a park sounds intriguing for someone loving nature, sitting on a wooden park bench in blinding daylight (even in a shade) is far from ideal. In the end, purpose build office is the most productive environment. And if there is one piece of equipment that can really improve productivity beyond a good chair and a desk (for the coming hot months), it's an air conditioner.
I think it's also worth mentioning that a desktop can be much quitter while being more powerful. To me, personally, that's a big thing. And one reason I don't like open offices. If I'm to be in an office building, I want my office. I want piece and quiet. And I like pacing while thinking. :-) Also, a secretary is nice. If there is one thing I really hate while working, it's calls. I like it best when calling me directly isn't really possible.