Why are we so strict regarding Tumat Niddah but not so with regard to Tumat Keri for a man?

Shalom U'verchah, The whole issue of niddah is a Torah prohibition, whereas the requirement to go to the mikvah for keri is a rabbinic requirement established by Ezra. This was not accepted by Am Yisrael and thus has not been upheld. Kol Tuv