Reproduction and defecation
The cloaca, which is the common opening of the urinary, excretory, and reproductive systems, is present in birds as well as in non-avian reptiles (and thus presumably dinosaurs), amphibians, and monotremes (e.g., duck-billed platypus). To answer your first question, yes, this condition does seem to be universal for those groups mentioned above.

To answer your second question, evolutionary history is as good a reason as I can think of. The cloacal system has worked well enough for >350 million years (in the case of amphibians).