The problem isn't necessarily speed, it's random access latency. What makes SSDs fast and "magical" is their low random-access latency compared to a spinning disk. The sequential-access read speed is merely a bonus.

Networked storage negates that significantly, absolutely killing performance for certain applications. You could have a 100Gbps network and it still won't match a direct-attached SSD in terms of latency (it can only match it in terms of sequential access throughput).

For many applications such as databases, random access is crucial, thus why nowadays' mid-range consumer hardware often outperforms hosted databases such as RDS unless they're so overprovisioned on RAM that the dataset is effectively always in there.