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Probably because it's easier to retain what the body wants than get rid of what the body doesn't want. What does your body want to keep from your Urine? Pretty much water and selective ions (Cl-, K+, Na+, Ca+2, etc.). Maybe a few other things, depending on how healthy you are. Now, what does your body want to get rid of in your Urine? Well, anything it ...

I would argue from an evolutionary perspective: not all kidneys are created equal. The mammalian kidney has a long evolutionary history and potentially a lot of phylogenetic inertia. Mammals do what they can with what they inherited from their ancestors. The metanephric kidney, which is what mammals have, is thought to have evolved with the first amniotes ...