The reason for having most instances use network storage is that it makes possible migrating instances to other hosts. If the host fails, the network storage can be pointed at the new host with a reboot. AWS sends out notices regularly when they are going to reboot or migrate instances.

Their probably should be more local instance storage types for using with instances that can be recreated without loss. But it is simple for them to have a single way of doing things.

At work, someone used fast NVMe instance storage for Clickhouse which is a database. It was a huge hassle to copy data when instances were going to be restarted because the data would be lost.