It seems that clouds balance their budget on egress charges... which leads to cross cloud communication being too expensive to setup multi cloud redundancy. Cross region redundancy is often too expensive too. Even cross availability zones is too expensive for some clouds and applications. (Cross region redundancy in a single cloud doesn't always work out, if the cloud has an outage on a global subsystem, or the broken subsystem gets pushed to multiple regions before exhibiting symptoms)

Additionally, moving your load to a different cloud can be challenging while one is down. It ends up being a lot of work that pays off for a few hours a year. For a lot of applications, it's better to just suffer the downtime and spend money on other things.