Based on personal experience, I believe that when confronted with the reality of a terminal diagnosis and chemotherapy is the available option, one's perspective often shifts—most people will choose to do whatever it takes to preserve their life.
Some forms of cancer are rather susceptible to known chemotherapeutic cocktails and have such high cure rates (in some cases over 90 %) with chemo that you'd have to be suicidal not to take the treatment.
Other forms can be significantly less susceptible, and the prognosis can be rather bleak even with the most effective known treatments. I can see how one might not want to suffer intensive chemo for a minuscule chance of survival or to extend life by a few months.
Also, n=1 and I don't know how my experience compares to others or to the average, but my experience with chemo wasn't that terrible. Definitely not pleasant, and there's a chance of side effects that could even be permanent. But not terrible. I've got the impression that the treatments for associated nausea etc. have also improved over the decades, and I didn't really have a whole lot of that.