They will and do something called acceptance testing, which is nothing but testing in prod.
You can't acceptance test "Google's search", because at a minimum doing so would require some kind of reverse proxy that is itself part of the system and can't be acceptance tested without...and turtles etc.
Another way of putting this might be that there isn't a good way to acceptance test "airplane manufacturing companies". There isn't a set of acceptance tests you can run to ensure that Boeing is performing to spec before having it build real airplanes.
It's clearly not perfect though, except perhaps if the test aircraft turned itself into a crater on the runway, there is very little one aircraft can do to effect the function of all others.
In software it's hard to get this kind of guarantee, a new delete with a missing where clause in your canary environment is going to take some time to clean up after (assuming you have backups etc).