You think it would be easy to lock up every animal that harms others? We have very different understandings of what the word "easy" means. As I said, the number of humans who have no understanding of morality is a tiny fraction of the number of animals in the world.
Not everyone, again, failing to achieve perfection doesn't mean we shouldn't do anything, but we should at least try to lock some of them, ¿would you agree with this?
If you can figure out a way to stop animals from harming others, while still giving them good lives, I am all for it. Simply putting them in a cell obviously wouldn't work, for the animal would not have a good life. It doesn't require very deep thinking to realize we simply do not have a viable solution to animals harming each other. Let's try to figure out a way to do so, but our greater concern should be in us to stop exploiting animals ourselves.
Imagine I am a slave owner, and you try to tell me I should stop exploiting people. If I respond by saying "hey, there are other people in other countries who engage in slavery, and you can't figure out how to stop them. Therefore, I am going to continue to own slaves". Would anyone take this reasoning seriously? Of course not. If we are concerned with exploiting humans, the first thing to do is to stop exploiting humans ourselves. Next, we should work on stoping human exploitation elsewhere. Even if we can't help others (positive rights), we should at least not harm others (negative right).