In my view, that's because our Leftist Brethren and Sistren rely on "feeliings", emotion, and ideology for guidance. Whatever is circulating in their so called brain at the moment is "truth." Therefore from their perspective there's no need to check facts, evidence, history, or read research studies.
THEY KNOW. Were you to ask hew they know, the answer would be something like, "Well I just know."
Common sense and evidence tells us that criminals are less likely to attack people who MIGHT be armed than they are to attack people the know are unarmed..