Chemotherapy wigs?

Ive always wondered about this and it is kind of random but here it goes. . . I don't understand why people who know they need to go through kemotherapy don't shave their heads right before and then get it made into a wig. I have pretty long hair and people are always telling my i should donate it to women who went though treatment. I don't have anything against it and i want to, but it would make more sense if thay shaved their own head because then you would have you own hair and you would never really even get bald, Right?? I think there has to be a reason i just don't know it. . any help