They are cheap. Cheap will always have a market.
If I'm hungry and damn near broke, I'll stop by there because there's one a couple miles from me along with a couple other FF joints. If neither of us feel like cooking there's usually a discussion of whether to go to McD's, BK, or TB because they're all within a few hundred feet of each other, all lined up. There used to be a "Kind of Pizza" Hut there too but it moved. No great loss.
I prefer to stick with smaller local places because they tend to focus more on quality instead of producing mass quantities. There used to be a small taqueria near here that had some good food but they shut down because nobody knew they were there. They were tucked into a little strip mall that hasn't had much traffic for over ten years and they didn't advertise at all.