Just curious, why would distributed be design requirement? Is individual machine failure likely in AWS/GCP? The only failure I have seen in region level issues which spanner or dynamo don't help with AFAIK.
Individual machine failure is not likely, but we're hypothesizing the need for multiple shards for high performance. So now we have more machines and so the probability of failure increases. So we need to add replication, but then we need to deal with data getting out of sync, etc.... As others have mentioned though, these issues only really become important at a certain scale.
