Because there’s a 5 to 10% chance the immunization failed.
If I told you there was a 5% chance a rollercoaster would fail and maim you, would you risk it? I’d rather not have to find out my immunization failed by being exposed to a sick person.
Read the studies. It is a much higher chance than that.
Vaccines are about managing risk. That 5-20% failure rate could lead to the same situation you have now.