Nearly every man you know has either directly experienced or witnessed this, not to mention messaging they get from society about it. Even when you hear women say they find it endearing when men are vulnerable, it's often still framed as a way of him displaying strength. He's "strong enough to be honest with his emotions or cry". If he's not emotionally expressive enough at the right times, he's "insecure in his masculinity (not strong)". It's also often with a backdrop of "it's special because he doesn't normally open up like that" aka he has to be strong 90% of the time otherwise instead of appreciate his rare vulnerability she'd be "man keeping" of he'd be "making her his therapist/mom". Women not being able to understand or empathize with this is similar to men not being able to understand the fears women have around safety or societies expectations about beauty and other feminine performance. Because they haven't experienced it, it's hard to imagine the magnitude of it and how it impacts your thinking and behavior.