Because "burn it all down and rebuild everything" is by far the easiest orchestration to create from a declarative config. And is almost certainly not what any technician is going to want to happen to their production system.
The clients and network have state. If you kill every service, and set everything up from scratch, your users are probably seeing an error message. Their connections all died and the retries failed while you were spinning up the new version of things.
Not when your infrastructure includes your data store, though. Or if it includes such things as your application endpoint. You may want to stand up the new endpoints and then drain over traffic, as an example. Not take an outage for a deployment.
Right, yes - sorry, I should have clarified that I didn't mean "tear down then set up", but rather "set up (new), cut over, then tear down (old)". But still - the service should be "created from scratch"!
