Question to Passengers

After a torturous 10 hour flight to Seoul yesterday, I have a question to fellow air passengers:

Why is it that when you bang your tray-table back into position violently, or use the seat in front of you as a lever to help you get up from your seat, or constantly push your feet onto the seat in front of you - why is it that you do not realise someone is actually sitting in that seat? And that specific someone might be trying to get some sleep?

Why is so difficult to understand that the back of the seat in front of you also has a front to it, with a person sitting in it?