> Why don’t the surrounding cells say ‘hey, you’re going crazy, stop!’?

A cancerous cell might develop a mutation that would reduce the amount of a protein on it's surface that would normally be recognized by immune cells trying to kill cancerous cells.

No two cancers are exactly alike on a genetic level, even though may have the same medical label. Each is a unique collection of many mutations that lead to a cell population that can replicate, invade, evade immune detection, etc.