AWS isn't for tinkerers and doesn't have guard rails for them, that's it. Anybody can use it but it's not designed for you to spend $12 per month. They DO have cost anomaly monitoring, they give you data so you can set up your own alerts for usage or data, but it's not a primary feature because they're picking their customers and it isn't the bottom of the market hobbyist. There are plenty of other services looking for that segment.

I have budgets set up and alerts through a separate alerting service that pings me if my estimates go above what I've set for a month. But it wouldn't fix a short term mistake; I don't need it to.