How so? I'm sure you take my point: that by indulging selfish wants, we can harm others. Private situations, do what you like; shared or social contexts, not so much.

Masks don't even work for many of the things people want them to work for.

Oh, hey! That makes them an even more perfect analogy. They help, massively, protecting others by removing many forms of transmission; but they don't prevent all transmission. Likewise, memory safety helps, massively, protecting others by removing many forms of security vulnerability; but it doesn't prevent all security vulnerability. And, crucially, not to get too far distracted from the original point: both are things we try to make use of where we can, not for ourselves, but for others and for the health of our overall communities. To really spell it out in words of one syllable: that's why people care about whether other people use memory safe languages.