> I mean, if you could download an update that would fix the wiring in your house, it would be much less critical that the initial installer got it right

As in my example, some random stranger needs to first find out your "house" (the vendor's software) is wired wrong. And this needs to happen for every "house" (every piece of software). While waiting for this to be discovered, your house burns down (hackers penetrate millions of devices, or perhaps just Microsoft Sharepoint that the govt is uses).