Oddly enough, in Haskell (as defined by the report), length is monomorphic, so it just doesn't work on tuples (type error).

Due to the way kinds (types of types) work in Haskell, Foldable instances can only operate over (i.e. length only counts) elements of the last/final type argument. So, for (,) it only counts the second part, which is always there exactly once. If you provided a Foldable for (,,,) it would also have length of 1.