Well for one, most can't. Exceptions do exist, and cross breeding does take place, if the animals are not separated on different continents. Lions and Tigers can in fact cross breed, but they are separated on different continents. Birds do cross breed, which is how many varieties occur, take Rosella parrots, they cross breed if allowed, and hybrids are generated. Any animal trying this would have to be nearly 100% genetically identical, otherwise no baby ← Just as well too given the Kiwis affection for sheep