It's not sterile. What's in there just doesn't grow if you simply put it on petri dishes, similarily to the gut biome or the bacteria in your stomach. They are adapted to an specific low-oxygen environment. Which means when you drink your piss, all bets are off, because the bacteria in it might like your stomach or gut, even when not growing on an agar plate.

This is also the reason why research in things like Heliobacter pylorii (a chronic stomach bug, causing all kinds of cell damage there) are so complicated: you have to use a bioreactor tuned to the specific conditions of the human stomach to even grow it, and research also has to take place in these, with very aggressive conditions compared to "in the open under the fume hood"