(not a doctor, this is a layman’s explanation)

What makes most cancer so deadly is metastasis which is when the cancer spreads from its original site to remote places all over the body. Each one of these metastases can grow into its own tumor and damage nearby organs or otherwise cause disease.

Take melanoma (deadly skin cancer) as an example. The original skin tumor is not life threatening by itself. However, if not removed quickly, cells from the skin tumor will metastatize and spread all over the body, causing secondary tumors to grow in other organs like the brain, liver, lungs, etc. Once metastasis has occurred, it is very difficult to kill each and every one of the secondary tumors, and death is much more likely.

Other cancers, like brain tumors that originate in the brain, are deadly not because they metastatize, but because it is very difficult to completely remove the tumor without destroying vital brain tissue.