The risk / reward ratio is just too high.
EDIT: Of course, I'll acknowledge that not everyone can be in the position of dictating what they use for work and requirements my force you to.
I don't see this mentioned often but the ergonomics of a laptop are rather poor, you'll end up with RSI much quicker that with a desktop. And this can become quite debilitating.
I don't think this is an issue for a couple of hours usage a day, but I would not want to spend all my computer time on a cramped keyboard with my head down.
The head down thing though is valid. It can be mitigated by having a stand or a monitor. I mostly deal with it by just taking a small break every now and then, although that's more related to sitting down than my head position.
A couple of reasons come to mind.
1) Price and value. With the exception of all-in-ones and the 2013 Mac Pro, desktop computers can be upgraded, repaired, and you get a lot more for what you pay than a laptop. Also a desktop machine usually lasts a lot longer than a laptop.
2) Flexibility. Laptops are now more than ever closed systems with soldered parts. Which means that (like an iPad) you don't usually have many options available.
3) Power. This is an obvious one, but to get performance you still need a huge laptop or a desktop.
I recon none of these points will make much sense if you absolutely need mobility, but as someone that has both a desktop and laptop I see value in both form factors.
Is that really the only place you use your computer?
I'm a consultant, working part time at an office, and I am doing a masters and generally just enjoy coding as a hobby. My use case is something like:
I develop on my couch, I develop at home, I develop on the train, I develop in the dining room, I develop in my own room, I sometimes write some quick code while on the bus during my commute, and I write code at my university. I also bring this same computer to work and use it to develop there.
Oh, and I also sometimes enjoy gaming with my friends at their place, with the same computer.
Why on earth would I restrict myself to a stationary computer when I can have a portable one that I can take everywhere I want with no hassle?
In my circles the only people that have a stationary computer are people that use it for gaming, and even then they still have a laptop on which they do everything else.