While I know anecdotal evidence doesn't mean much, I personally felt a lot of empathy towards the animals when I took my son last year. I think it is hard to read every plaque in front of the exhibits and not be sad. Almost every plaque says something about how the animals exhibited are struggling in the wild due to everything humans do to the planet, and each one gives suggestions on how we can improve the world. I may be a rare case though because I can understand and acknowledge the environmental impact we have as humans, and I actually read the plaques. I think you may be right about the majority of people who go to the zoo, but I do feel like it is effective if people engage with the lessons that zoos try to teach.