Why does every conversation about women's issues end with "not all men"?

Every time women talk about harassment, safety, sexism or everyday experiences, someone inevitably jumps in with "not all men." But don't everyone already know that ? The conversation was never about every single man on earth.

What frustrates me is how quickly the focus shifts from listening and understanding to defending male feelings and reputations. The issue being raised suddenly takes a back seat to reassuring men that they aren't personally being accused.

To me, "not all men" often feels like a cover to hide behind so nothing actually has to change, be challenged, or be addressed responsibly.