system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't you just cut off cancer cells?

I know there's a reason, but I don't know what it is.
gpt
A lot of tumours ARE surgically removed. But there's several big issues with that. 

Surgery may leave small, microscopic pieces behind that could grow back.

Cancers spread, so a tumour may have already sent out metastatic cells to other parts of the body.

Most of your organs are necessary for you to live and there may be no way to remove a tumour without removing all or most of the organ.

Tumours may be in a surgically inaccessible area, ie the deep interior of the brain or on the brain stem, or inside the bone marrow (leukemia)