This is not entirely accurate. If you want to forego the ability to support cross shard transactions then sure you can run consensus independently on each shard. But I don't think that is what Spanner/CockRoachDB do. In addition, even if you want to limit transactions to a a single shard, the shard replicas need to be in different datacenters/AZs if you want to remain available when a DC/AZ goes down.