Maybe I'm oversimplifying things, but why haven't these companies distributed their compute resources across various facilities and cloud providers, enabled instant failover, and tested this before outages like these?
It costs engineering time to do so. Time that could otherwise be used to build features, better protect against more common failures, attract users, etc. Amazon probably has ~5hrs/year of complete failure of a region. Figure, conservatively, it would take 3 months of engineering time to protect against that, plus a 'continuing' cost of 1/2 a week per month to maintain that protection. You'd also have to (at least) do…
Instead, to go with amazon you have to architect for Amazon, not counting on your ECC instances to be up all the time, accounting for their local fast storage going away, or accounting for how EBS, which is persistent, is slow.
The alternative is, get the enterprise version of Riak, purchase dedicated nodes in two data centers, tell them about each other. (no engineering required.)
If engineering resources are the most precious commodity, it seems AWS is the more expensive option.