If it was the Nazi's goal to eradicate certain ethnicities or religions, how come people still made it out alive of concentration camps?

Just to clear any potential confusion, in no way at all do I mean to imply that people shouldn't have made it out of concentration camps. What made me ponder is a documentary I was just watching about Mengele and medical experiments on prisoners.

Essentially I was wondering why some people survived for so long in Auschwitz (still acknowledging that a vast (?) majority died there)? As far as I can understand, jews, gays, political opponents, communists, gypsies etc were brought to concentration camps with the explicit intention to exterminate them and their kind...

So taking aside the fact that men were kept alive for slavery purposes, what motivated the Nazis to keep others who were of no direct "use" to them? For example, why did they keep feeding them? Surely it can't be out of compassion or about having qualms about their prisoners' misery... Were the gas chambers running at full capacity?

Or am I just missing something?

Thank you in advance for your answers.