Because that certainly rolls off the tongue. And if you think about it, we can be pretty sure that respect and agency actually are species specific ideas. You can debate what species, sure, but all plant matter constitutes both life, and a wide variety of species. I would argue that grass, of any variety, is not deserving of respect or agency. You can plant it, mow it, step on it, whatever. It’s grass. It doesn’t care.
Likewise in the animal kingdom, I have a gnat problem in my house. I don’t think it makes me one of history’s greatest monsters to leave out apple cider vinegar traps for them.
What’s attracting the gnats is my cat, who I certainly believe has more rights then they do, but he still has to go to the vet and get his checkups, regardless of his personal feelings on the matter.
And, of course, humans have rights that our closest kin don’t. We don’t let other primates vote in elections, and I believe that’s a wise and fair decision, as they do not appear able to understand the issues, or even what it is that they’re doing.
In conclusion, an appeal to humanity, or human rights, is a perfectly valid one.