Because that makes him a character that kids and adults going through chemotherapy can look at and see themselves in as strong and powerful heroes who can overcome every opponent with ease.
I know there’re plenty of other bald badass characters in fiction. But all of those characters shaved their heads voluntarily. Saitama lost his hair, and he lost it in the process of becoming strong. That’s why I think he’s a character people going chemo can look up to.
