I don't know how many times it has to be said: masks don't protect the wearer. They protect those around the wearer. If I wear a mask and you don't, you're protected from me but I am not protected from you.
Think of it this way:
If you and I are standing next to each other and neither of us is wearing pants, and I start to pee, you're going to get my urine all over your bare legs.
If you are wearing pants but I'm not, your pants will keep off a little bit of my urine, but you're still going to get most of it on your legs.
But if I'm wearing pants, my urine is going to get soaked up by my pants, and you will stay nice and dry.