No one really cares enough about hobby developers as a customer segment to rebuild the billing infrastructure to make this possible. Scalable billing at huge scale is solved at the cost of latency and being "eventually correct" (unless it has changed). To add a price cap feature, eventually correct isn't enough and then you ask yourself who would actually use it and you have to scroll really far down your list of biggest customers until you get to someone who wants it.
The problem with not caring about hobby developers is that it means developers won't be as familiar with your cloud environment when time comes to pick one for the next "real" project.
I would also expect a price cap feature to be useful for experimental/no-approval-required projects at work. In fact, if I ran a cloud project for work as a small team-internal project, a cost explosion would become an even bigger bureaucratic nightmare than if it happened at home.