"why is it that people who commit war crimes are not prosecuted?"

It's a matter of sheer power, that's all. "Might makes right." The only time high level war criminals (like heads of state) are prosecuted for war crimes is when they become prisoners of a conquering power, following a military conflict....or....

...sometimes a few of the small fry (ordinary soldiers) will go on trial as an exhibition to mollify public outrage. They are then sacrificed, but their top commanders and their political leaders go free.

And so it has always been. Might makes right is the functioning rule of international politics.