Are we quite sure that “all flesh” (e.g., Gen 6:12) does not simply refer to all of humanity? Why think it refers to all animals?

Verse 17 makes this abundantly clear: “And, behold, I, even I, do bring a flood of waters upon the earth, to destroy all flesh, wherein is the breath of life, from under heaven; and every thing that is in the earth shall die.” There is “the breath of life” in all of animal life, and of course all of animal life would be destroyed in a worldwide deluge.