You probably would agree that reducing suffering inflicted upon humans is generally speaking a good and ethical thing to do, right?
To understand why many people think the same re. animals, perhaps trying a different starting point might help: why single out humans? There's fairly good arguments (https://www.animal-ethics.org/the-idea-that-only-humans-are-... and the linked papers are a good resource) to be made that many other species have a similar capability to have positive and negative experience (sentience) which are not just citing 'common sense' or similar weasel-words, instead actually looking into the neuroscience of pain and other suffering.
To understand why many people think the same re. animals, perhaps trying a different starting point might help: why single out humans? There's fairly good arguments (https://www.animal-ethics.org/the-idea-that-only-humans-are-... and the linked papers are a good resource) to be made that many other species have a similar capability to have positive and negative experience (sentience) which are not just citing 'common sense' or similar weasel-words, instead actually looking into the neuroscience of pain and other suffering.