
Reusing the lining is different for us rather than mammals. Mammals do not menstruate like we do, they reabsorb the lining. However, ours is so thick and constructed differently that it makes it less of a cost of energy to expel it rather than reabsorb it. A lot of the human body/animal bodies are built on the notion of “what is less costly to do”
Which is a theory for death as well - as time goes on our bodies get damaged more and more, DNA strands become shorter as we age due to the replication/division process, and it is more expensive overall to keep maintaining an organism that is worsening. Our bodies just don’t handle it because we aren’t built to last, we are built to be temporary so that we can grow, survive, reproduce, and then die. It sounds bleak but it’s actually quite beautiful in a sense. It is this way for the sake of