That's understandable if they use a desktop PC most of the time, and a lot of people fall into that group.
Even with the clever "palm detection" (or whatever it's called) feature that is built into most trackpads nowadays, the very location of the trackpad on most laptops makes it too easy to click on something you didn't mean to. It's much easier to keep the focus in a <textarea> if you just type on a standalone keyboard and the mouse is 14 inches away.