This is slightly inaccurate. Pan-cancer treatments are rare but do exist, for example checkpoint inhibitors. They just work differently on every cancer and have different outcomes for each type of cancer. The single biggest factor right now on treatment outcomes is how quickly you find the cancer, because surgery early on is the best treatment, usually followed up with chemotherapy. These "miracle" treatments really are most impactful for late stage patients, especially those that aren't very expressive which means it's harder to find something to bind to the cancer cells and kill them.