Because Vaccines DO NOT CREATE A COCOON around your body so virus material can not waft into your respiratory system.
I was vaccinated for small pox before age 6. After age 8 I got inhaled a load of virus during a 8 hour train ride and smallpox got me very sick. But because I was vaccinated and my immune system was made familiar with the protein signatures of virus, my body was able to fight it off and I recovered. Mortality rate from smallpox is higher than 30%.
So, I do not wish to be in vicinity of a non-vaccinated person because they are more likely to carry higher level of covid infection and thus exude bigger loads of virus in air. Whereas vaccinated persons will most likely result in a mild asymptomatic infection and wont be coughing and exuding bigger loads of virus material.
When did you catch this case of smallpox?