This is a bit like asking, "Why do firefighters wait until there's an alarm to send the fire truck?" If cancer is caught early and the patient has a good chance of recovery and survival with the least aggressive treatment, then there's no reason to bring out the big guns which usually come with bigger risks.
LaxBedroom t1_ja47d6r wrote
Reply to ELI5 Why do doctors wait for cancer to progress to a further stage before prescribing certain treatments like immunotherapy? by JustMe182
This is a bit like asking, "Why do firefighters wait until there's an alarm to send the fire truck?" If cancer is caught early and the patient has a good chance of recovery and survival with the least aggressive treatment, then there's no reason to bring out the big guns which usually come with bigger risks.