Sorry to be pedantic, but a minor correction. Songs like Frere Jacques which repeat the same melody are actually canons, not fugues. A fugue seems to repeat the melody, but starts from different notes in the scale, therefore the end product is not the same.

On a more serious note, why are bottom up solutions better? As you say, there are many examples, but is there a credible scientific explanation as to how this is possible? You seem to make this declaration without demonstrating it.