As a follow up, how can Keri by derabban but Niddah is deoraita if both are explicitly mentioned in Parashat Metzora (both require sacrifices and mikveh immersion)? Additionally, if we are strict for Niddah because we dont know how to distinguish between Niddah and Zavah, why do we not do the same for Keri/Zav? It seems if it is mentioned in the Torah for Keri it is on equal footing as Niddah.
Shalom U'verachah, The Torah speaks about baal keri in relation to tumah and taharah, laws that are not practiced today. However, Ezra added an injunction that baalei keri have to go to the mikvah even in order to pray. This was not accepted by the people and thus today we do not need to do through a process of taharah for baal keri. However, the laws of niddah are not only in connection to tumah and taharah, they are also in relation to relations between husband and wife, thus there are still relevant today even when we are not particular about tumah and taharah. Kol Tuv