Because the risk without vaccination is greater then the risk of vaccination
You ever seen someone crippled or dead with polio?””
The risk-benefit equation is a universally applied concept, for nothing is without risk, and the risk of adverse effects from vaccines is neglible compared to the potential devastation of an infectious disease epidemic, or worse yet, pandemic. The Spanish flu killed 500,000 young people in the US and at least 50 million worldwide. Real risk comes from dricving an auto, drinking alcohol, overeating, farming, coal mining, logging, etc.
We all task risks.
We all guard against risks.