Doctors don't typically let cancer progress without treatment. They treat the cancer, based on how far it has progressed and whether it's responded to treatment.

Early on, a cancer is contained in one spot, and it's almost always simpler to cut it out and be done with it. Maybe, follow up with a little chemotherapy (possibly localized to the spot) to be sure.

If swinging a scalpel seems like a bad idea (maybe in a sensitive part of the brain or parts of the neck), then radiation might be a good choice. Radiation is often also used to destroy bone marrow for blood cancers that are addressed with a marrow transplant.

Once cancer spreads, then surgery starts to be less of an option (you can only cut someone up so much, and you'd simply be playing "whac-a-mole" trying to get the tumors), you typically look at chemotherapies that go all over the body. They can be very tough as a treatment, but also effective.

Those are all well established and understood therapies that any hospital that treats cancer patients will be able to provide, and the doctors know how to use all those things to get the best possible outcomes.

Immunotherapies tend to either be very specific, so only usuable in certain situations, or they are personalized and need a facility that has the technology and people to implement them. Many are also novel and still being tested, so they are only available as part of a clinical trial, and a person needs to qualify and the trial needs to still be accepting new patients. Being new, it's also VERY expensive and if many insurance plans only cover it if other treatments haven't worked.