My suspicion has always been that people would probably rarely chew gum from single small packets if it wasn't there where you line up for checkout. Instead, they'd buy the same-size packets that come in bunches of 5 or whatever, and usually cheaper per pack anyway.

You often see batteries there along with them with the gum and chips/crisps. At least batteries practical sense - the last moment things people tend to forget, but actually do need or want.