Ugh. I know, and if I hear or read "what-about-ism" one more time I'm going to explode. Imagine if the public acted this strongly toward all wars and injustices. We might actually create the change we need.
I have a few theories on why we act like selfish, know-it-all jerks, but humans are naturally empathetic. If you discuss an issue with someone they will provide every solution they can think of, but when it comes to challenging a belief people switch off.
We feel criticized when our government/country is scrutinized because nationalism is embedded into our identity from childhood. Speaking negatively about the US feels like a personal attack, and acknowledging all the horrors the US has committed is a lot to process. So, people subconsciously choose ignorance and denial, but I'm pretty sure they can't escape the guilt and anger.
Then there's the media. It knows how to gain our attention and use our emotions, but it doesn't teach us to think. We are an emotionally impulsive culture. Capitalist Realism has a great chapter on the subject, and I really enjoyed the book. The author describes and explains society's behavior under capitalism, and the details offer a lot of clarity.