[derekperkins]

That introduces different problems. Applying MySQL replication logs is single threaded by default and generally doesn't scale as well as the master, so it becomes increasingly difficult to keep replicas from lagging.

As your database gets into tens or hundreds of TBs, now backups and restores take hours or days to complete. Any DR scenario becomes an existential threat.

For large tables, schema changes can take weeks. This often results in developers doing their own custom table sharding strategies, even though they might still be on a single machine.

I'm not saying that you can't architect around these problems, but operating a db at massive scale on a single machine isn't the obvious win that it seems to be.