Why in the world would they?
Can you not tell the difference, yourself, between still water and human urine? One is a nutrient, the other is a bodily waste. What about on the basis of relative taste, colour, smell? Urine is full of mild toxins, which most pond or tap water is not. Urine contains lots of urea, a nitrogen-heavy chemical, which is great for plants but not beloved of aquatic creatures (like, er, mosquito larvae).
Also note, fresh human urine from a healthy person is normally sterile; it has a very low bacterial count, probably lower than from tap water -- and certainly a lower microbe count than stagnant still pond water. This is relevant, because mosquito larvae (what hatch out of their eggs) live on "infusoria" -- a generic term for a variety of microscopic creatures. Infusoria include ciliates, euglena, paramecia, protozoa and unicellular algae.
So infusoria don't easily live in human urine, and thus mosquito larvae couldn't find enough food in a pool of human urine. Assuming that that urine pool was otherwise large enough to be attractive and no stagnant water could otherwise be found for mama mosquito to drop her eggs into.