Comment by tsimionescu

Well, let me ask you this: why not stop the computer when a program encounters a bug? Why not the whole cluster? Theoretically, an attempt to write to a null pointer could happen because you have corrupted a database or file and the entire system is now in an unreliable state.

The answer is that just as a process has some degree of isolation from other processes on the same system, and from the kernel, similarly components of a process can be well enough isolated that only the specific component that encountered the bug needs to be stopped. This is never 100% safe, but how safe it is depends greatly on the technology and architecture.