Because it takes far more effort to carefully rebut a slur than it does to make it, and if you let it stands as is, you've lent it credibility. We all intuitively understand this, which is why overt arguments in favor of crimes against children (which arguments do occur!) are instantly flagged off the site; we don't entertain them or carefully rebut them, and practically no one on the site expects us to. It's just that a noisome faction on the site does expect us to entertain slurs against women.
“In this endeavor, deniers focus on many minor and obscure details and leave out crucial context. It takes them little effort to formulate a wrong assertion, but it takes historians a long time and a lot of words to refute one. Our early attempts to engage on these points have shown that length and nuance do not play well on the internet and do not interest the deniers. The point of JAQing off is not to debate facts. It’s to have an audience hear denialist lies in the first place. Allowing their talking points to stand in public helps sow the seeds of doubt, even if only to one person in 10,000.”
