Because that wouldn't have any effect. The point of such a poster is not to be politically correct it is to hopefully hit someone in that part of their brain that will make them stop being a piece of shit. This wording has a much higher chance of that than a completely impersonal unrelatable "don't hurt them they are humans too." That's too broad. You have to get closer down to something they can visualize. Something that isn't just a concept but something they have experience with.
This way it has a higher chance of getting to someone. Still not a high chance at all but much higher comparatively to what you suggest
To those of you downvoting me think about it this way. If you think of the word human no one specific comes to mind. It's closer to a concept than a physical thing you can see. If you think of the word daughter, mother, or sister you will think of someone specific. Someone you most likely love. Someone that you wouldn't want harassed and that fact is why it is more likely to trigger someone to have that link in their head get built to make them think about the fact that they wouldn't want their loved ones treated the way they are treating other people.
This way it has a higher chance of getting to someone. Still not a high chance at all but much higher comparatively to what you suggest
Welp time to wait for my slow inevitable death in a few minutes