EBS attaches a single block storage volume to a single host[1]. S3 Express is a service-based object store. Apples and oranges.

[1] Yes, I am aware of multi-attach but this introduces a scaling bottleneck and requires a fairly exotic setup.

1. You'd need an access/authentication layer on top of that.

2. Variable throughput may be a concern.

3. You may have availability concerns.

Yes, EBS is the gold standard but managing a EBS to scale up and down instantly, be available to multiple instances, lifecycle management, managing replica, switchover etc. are definitely not easy. And EBS are bad choice when throughput needed is very spiky.