You can totally build a GCP function that will tear down your site if a billing alert hits the pubsub bus, but you can also set billing caps for a bunch of the APIs.
> None of them have an equivalent of a stop-loss.
Cynically, it's because they want to take your money. Practically, it's because a company that breaks your thing because you didn't pay, even if you asked it to, just isn't going to be a popular move.
It's probably easier to make it so they can set up alerts and have a human on their end evaluate what steps are worth taking to cut costs while maintaining important availability.