It's been a while since I've read this, but wasn't the point that they couldn't leave? Everyone gets caught up in the inescapable and infinite nature of the spiral, which twists space and time. In the end, everything spirals towards death and horror, the center. I didn't feel an explanation for the origin of the spiral was needed at the time I read it. Because of its theme, the ending was a perfectly logical conclusion, as it takes place in a world ruled by the spiral (including time and space). Of course, if you are not a fan of the metaphoric and open endings, this one was indeed horrible. Personally, I loved the ending.
Plus, if nothing else, it was hilarious.