Why do you say it's not supposed to happen? It's the predictable outcome of not having unlimited scale. The hyperscalers are big, but to be able to accommodate a tectonic shift in demand in a very compressed amount of time is very costly--costs that will be passed to their customers--if it can be done at all given current constraints. Today's hardware crunch, even for hyperscalers, is very real; customers are hoovering up spare capacity like never before.
As a former guidance counselor (specialist SA) for AWS, I regularly told customers that they shouldn't expect to be able to pack up and relocate into a new AZ once an incident commenced. They rarely listened, as it was often deemed too costly to maintain AZ redundancy (let alone regional redundancy) in advance of an incident.