Why do people not understand animal rights?
People are not stupid. They do understand and most of them saw footages from slaughterhouses.
I have been involved animal rights activism in many countries. Generally speaking, people have a different attitude towards the issue based on their social, political, cultural and economic situation.
In the western world, mainly, people have comfortable life compared to the second and third world countries. Therefore, statistically, there are more ethical vegans/vegetarians and people who aren't basically, choose to ignore those footages but they are well aware of the problem.
When it comes to second and third world countries, they got tons of problems to deal with, especially with human rights and usually, when you talk about animal rights the answer you are going to get "We got other serious problems than animal rights.". In those countries, some people daily fight for survival and they eat whatever they find, others, have so many financial problems that they cannot afford to think about other issues. Moreover, the governments don't take well any kind of protest, people afraid of the protesting for their own right and when you show up and talk about animal rights they would laugh at you. Nevertheless, I know many vegans/vegetarians in these countries and they still managed to stay like that many years.
Overall, many of them know and choose not to act and ignore, others have neither comfort nor time from daily problems to get involved.