You are right, chemotherapy means treatment with drugs, its not specific to cancer. However, for some reason it has come to mean "cancer treatment", why I don't know, but there's nothing we can do about it. To try would only cause distress. Many years ago my friend's dad went to the dentistwith an infection. They said he would need a course of antimicrobial chemotherapy ie. antibiotics. This is a perfectly correct thing to say, however he just focused on chemotherapy, so by the time he got home he thought he had cancer and was going to die! His wife was a nurse, and a quick call to the dentist sorted it all out, but he did have a bad time over it.
Its just the way language develops. eg many people in the UK , if they are going to use a vacuum cleaner would refer to it as hoovering or using a hoover, regardless of whether the machine has been made by Hoover or not.
Still, getting back to your question, I don't know why chemotherapy is only used in relation to cancer,it just is.
Don't try to keep up with the Joneses - bring them down to your level !