It doesn't. But I don't think that's an issue considering providers wide outages are exceedingly rare. Even last year's massive AWS outage was isolated to us-east-1.
A providers outage can also be named a price hike, a change in license or a security breach. In that case you might want your data out of the provider asap.
My experience is, that a company who decides to use AWS (or whatever) only decided to use one specific vendor and would need to reevaluate every other vendor the same way.
If you can't trust AWS enough, i wouldn't use AWS.
if the software is cloud agnostic, wouldn't a simple copy operation between the cloud providers suffice?
>security breach
if you want to reduce your attack surface, copying your data across multiple providers is counter-productive. instead of having to hack your specific cloud provider, the attacker only needs to hack one of many cloud providers.
In some cases a copy might not be viable, especially with price hikes. If AWS increased storage and outgoing data costs greatly then exporting all your data will most likely cause a much larger bill.
Splitting your data across multiple vendors protects against all kinds of failures from all sides (including the vendor simply closing your account for no reason)