Even a naive sharded approach requires coordination if you want a scalable solution. Where do you send keys when there are 4 shard buckets? What happens when we want 5, or 8, or 32 shard groups? Just yolo turn off all writes, toggle some config and turn everything back on? Not great. You've changed the shard selection of a key from 2 to 11? What happens to all the previously persisted keys that were written to 2? The answer is unsurprisingly complicated, and depending on the system you're writing, this can be a really hard to solve against if you haven't prepared to grow (or shrink).