Because it would tear down the entire stack? If you're a real business, depending on servers to be up and data to stick around, then it makes absolutely no sense to have machines shut off and volumes deleted if you hit some arbitrary marker. There's nothing you get for free in AWS (besides the Free tier, and not many people are running their entire, highly profitable business on that) and the only solution to "not spend more than $X per month" is to literally shut down and delete things.
I'd love to hear use cases where legitimate businesses, who make money off of the products or services they offer, can literally afford to have their business just stop working. It sounds totally contrived.
A CloudWatch alarm on a billing metric could also be used to send you SMS, email, hell even call you if you want to wire a webhook up to a quick and dirty twilio app (via SNS)
In fact, we use an SNS->Slack gateway running on a free Heroku dyno to get out alerts in a Slack channel (which is pushed to phones/etc), along with other CloudWatch alarms related to performance.
Honestly, this issue of "I don't have any visibility into what I'm spending" is a waste of energy. You do, and you can have AWS bug you as intensely as you want with updates as frequent and as urgent as you need
I'd love to hear use cases where legitimate businesses, who make money off of the products or services they offer, can literally afford to have their business just stop working. It sounds totally contrived.