The true explanation, in my opinion, is that bakers just don't like tens. They prefer dozens, or more generally, multiples of three and four, notably four, six, eight, and twelve. These quantities lend themselves to compact packaging--three rows of four, two rows of three, two slabs of two by two (e.g., hamburger buns), and so on. Ten lends itself only to one row of ten or two rows of five, which are seldom compact shapes. Therefore, the baking mind-set--and here's where we start getting into anthropology--is such that you instinctively regard ten as an unwieldy number. When the pioneers of bun baking were trying to figure out how to package their product, they probably figured what the hey, eight makes a squarish package, so that's what we'll go with, without even considering the unique circumstances that made ten more appropriate.