I assume they used all their fuel and energy for the war effort. They built POW camps because they cared about their prisoners. If they didn't care, they would have just murdered them all. It's hard to fathom how many bullets they fired on Normandy or in Russia. I don't recall one video or documentary saying they were low on bullets. For that matter, I doubt they were low on bombs. If they wanted to kill everyone in a POW camp, they could have easily fire bombed the camp like the US did in Japan to Tokyo. It would have been an absolutely tremendous waste of forces wasting time gassing and burning POWs when fire bombing would have resulted in the same result and only took ten or twenty minutes.