Really? Is that the best that you can come up with? Slaughterhouses are the last thing anyone thinks or cares about, but it's certainly not for something petty like opaque walls, or children not being taken to slaughterhouses.
It's because the creatures being taken to slaughterhouses are inferior beings. It's because they aren't people. It's because they wouldn't be capable of having a single conversation with a human about the slaughterhouse or their beliefs about it. Until other animals are capable of advocating for themselves, like even a child knows how to do, then they will contjnue to get what they deserve. No matter how many class trips you organize and glass windows you install.