Consider the same question for basically every nonperishable you purchase - why are you not buying in large bulk and saving? I can think of a dozen items I could buy in bulk and save money on that I currently don't.
Actually, now that I think about it, why don't I buy in bulk? I totally could. Why don't people in general buy in bulk?
Anyway, it's the same answer for your toilet paper as it is for your razors, or your shampoo, or your paper towels. I don't apparently know that answer, but I assume that's why people don't do what you're saying.