Because the author puts those words in their mouths -_-
Or it has to do with the fact that yukkuris have a lot of unfair advantages in that they dip into the universe's library of knowledge bit by bit and the only reason why they do not get to do great things about it because they are taking it too easy or because their biology or own self regulating ecosystem does not allow them to upset the balance of their respective yuniverses (because the authors wont let them because of reasons)
they are obviously not looking for a doylist explanation, but honestly there may be no in-universe explanation