What is it with maths?

I have been enjoying reading the responses to my homeschooling meme. One thing struck me. What is it about maths? Why is it that for so many people the resource they wish they hadn't bought is a particular maths curriculum?

The Lilting House - Right Start Math
Karen Edmisten - Singapore Math
Cirque de Moi and Adventures With Books - Saxon Math
Dumb Ox Academy - MathUSee
Myself - Miquon Maths
... and most thoroughly, A Room of My Own's "vast collection of math curricula"

Perhaps I should have answered the "one resource you wish existed" question with a foolproof maths curriculum, guaranteed to be a good fit for all families.