The need for consistent, highly available persistent storage  in a distributed system with a heavy write workload <i>vastly</i> complicates engineering effort. You&#x27;re fighting fundamental issues like the CAP theorem. When you can&#x27;t &quot;just use Postgres&quot; everything else gets a lot harder. Companies that have this problem tend to have a <i>lot</i> of engineers: Facebook, Google, Uber, etc.<p>Disks are slow. If you don&#x27;t need to wait for disk, then you can handle a great deal more req&#x2F;s per node, which means the point at which you need to become a distributed system is further away. Without the state synchronization problem, converting to a distributed system consists of little more than adding a load balancer.<p>Even in an environment that needs synchronized state, the services that can be stateless are much easier to deal with (and you try to make as much of your infrastructure stateless as possible).<p>Storage costs are not relevant compared to the costs of engineering distributed systems.