For the same reason as to why he was put in a manger between two animals. Isaiah 1:3.
And that, boys and girls, is why the ox and donkey are included in Nativity Sets (which we’ll see all over the place in a couple of weeks). There’s also a verse about this (although it only says “beasts” or “animals”) in the Septuagint version of Habakkuk.
Interesting links!