Why human endometrium doesn't stay constant?
One hypothesis is to protect the uterus and oviducts from sperm-borne pathogens.

Also Beverly Strassmann has proposed that the energy savings of not having to continuously maintain the uterine lining more than offsets energy cost of having to rebuild the lining in the next fertility cycle, even in species such as humans where much of the lining is lost through bleeding (overt menstruation) rather than reabsorbed (covert menstruation).