Fair, but if all of those 5 banks bought their own servers and put them into a datacenter that happens to be same one because it's the fancy one that meets all the bank regulations and required certs, that datacenter going down would still make 5 banks go down.

You would got some resiliency by fact only datacenter related issues (power, internet access) would affect it and not say "cloud company throwing AI at their code that proceeds to explode" (which already happened), but still, if you want to fix that, cloud or not, you have to have multiple locations/regions and that's both costly and difficult.

You misunderstand.

This is a provider outage, but the services running on them by the customers (that went offline) were not designed or setup to be multi-region.

The infra outage is GCPs fault, but application outages from customers are caused by their architecture and decisions prior to the outage.