Why not add it directly into the interface where you configure the site to begin with? It's a pretty reasonable ask.
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.
> 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.