Why is that?
What is "God's design"?
Leviticus 19:19
You shall keep my statutes. You shall not let your cattle breed with a different kind. You shall not sow your field with two kinds of seed, nor shall you wear a garment made of two kinds of material.
Only domesticated, by man, animals are routinely changed through breeding crossouts. God gave instruction, but we often have a better idea, we think. In the rest of God’s creation, even though they may be quite similar, like cardinals & robins, or grizzly bears & black bears, or bass & trout, they do not crossbreed. Looks like a plan. I called it God’s design, since everything is from Him. Further, I believe I said, my opinion.