Core AWS services use it too. Even if you are hosted in another region, you can still be affected by a US-East 1 outage
I was surprised recently when setting up cloudfront with aws certs that it forced me to use us-east-1 to provision the certs.
Bingo. This is the one most people don't know about.
The idea would be to actually load distribute between different cloud providers.
But even then , the load balancer needs to run somewhere. Which becomes a new single point of failure.
I’m sure someone smarter than me has figured this out.