It's simple...
Masks aren't magic. Masks won't 100% prevent you from getting the virus. They won't 100% prevent you from spreading the virus. They will help, but no guarantees.
So the goal isn't to analyze this from the "me, me, it's all about me" perspective of "should I wear a mask based on my own personal risk profile?"
The idea is if we ALL wear masks, then we'll inhibit the spread greatly. And if we inhibit the spread greatly, we'll protect those vulnerable populations much more effectively than telling them to go hide in their homes--because they need food, they need to go to doctor's appointments, etc. They're going to interact with the public to some extent, even if they're trying to minimize it.
It's about helping the rest of us, not just yourself.