The startup costs, ongoing maintenance, and organizational costs of mitigating this kind of risk have to be considered with an evaluation of the likelihood of the event occurring. And other things the company could do with those same resources.
Do you have a fully equipped backup home somewhere far away you keep stocked with consumables and perishables in case where you live is destroyed by a meteorite one day? If not, why not?
Yes. I have some friends and family members who live anywhere between 1 mile and 10,000 miles away, in both same country and other countries, that I can turn to provide me with food and shelter if my home is no longer available for some reason.
Regardless of them, I can take shelter short term in a hotel or other rental facility, as I have sufficient savings that I'll continue to be warm and fed.
The comparison doesn't hold up. A business cannot just reform overnight if critical dependencies disappear. A person can.
This is equivalent to knowing you could get a hotel room or crash with a friend, instead of maintaining a whole second home yourself.
But my company does everything we can in order not to have a single point of failure, however unlikely it is to fail. Because low probabilities of something still mean it can happen, and you don't want to allow that if you can avoid it.
I don't understand why you would allow this situation to arise, though. What are you waiting for ordering those servers right now, while Amazon is still working?
If you require hundreds of physical servers, it's basically guaranteed that on the long term it would be cheaper to host them yourself than let Amazon do the job, with the risks that that entails.
It's fine to take this kind of risk while you're not sure if you're going to operate long term at all, but once that's established and if you're not a small shop, depending absolutely on one company is unforgivable.