You shall observe my statutes (chukotai)
...Rather, chukim are like the decrees of a king who decrees it with his authority without revealing their purpose to the people... But all of God's laws have a correct reason and a perfect purpose.
And the reason for kilayim is that God created species in the world - in everything with a soul, in plants, and in everything that moves - and gave them the ability to reproduce, such that the species survive with them forever, so long as the Blessed God wants the world to exist. So God commanded that they be able to produce according to their species, and that they never ever change. As it says, when describing all of them, "according to its species" (Gen. 1)...
And one who crosses two species together, changes and denies the work of Creation, as if they are thinking that the Holy Blessed One did not perfect the world completely, and this person wants to help with God's creation of the world by adding creatures to it.