I had the same problems years back when I first tried AWS; I mostly just learned everything by rote.
Recently, though, I tried setting up a simple home "VM lab"—basically a two-node cluster of regular workstations, running bare-metal hypervisors (e.g. Xen, VMWare ESXi, etc.)
When you do this, suddenly everything IaaS providers do comes into stark clarity.
For example, you realize that with even two nodes of scale, that sticking a lot of storage into each VM node is expensive, especially if you're not going to use it all on each node; it's a lot cheaper to have diskless VM hosts (save for an SSD for swapfiles), and stick the disk volumes on a SAN, connected to the VM hosts by something like iSCSI. (This then gives you other benefits, like disks that are copy-on-write clones of other "template" disks on the SAN; the ability of VMs to "fail over" to another VM host (and thus the ability of the host to gracefully restart without killing its VMs—at least, from a user perspective); "free" snapshots with rollback; etc.)
EBS is exactly such a setup—the perfectly-sensible setup for any VM cluster operating at scale. It's the other setup—local disks in the VM hosts—that's insane and doesn't scale; everyone is just familiar with it because most people's experience of providers is with the ones just getting off the ground who are before the ROI intercept.
Or, for another example, when running a heterogenous VM cluster, you immediately realize that the unit that the VM CPU resource commitment is measured by on a VM host is a "vCPU"—which is not exactly one reserved physical core (since it can just represent a hyperthread, and can be overcommitted when other VMs aren't using their resources) and is not the same on each host, since each core operates at a different speed.
The closest you can really get to getting a useful unit of CPU allocation, then, is by rounding off the host differences to integer multiples—such that you could say that one vCPU on host A is (roughly) equal to two vCPUs on host B. That unit, whatever you call it, will then scale up over time, as you phase out previous generations of VM host hardware. That's an ECU.
Smaller hosting companies can get away with telling you what CPU you'll be running on, because smaller hosting companies are new enough to have homogenous hardware, and small enough to not attempt anything like live VM migration. A company that has both heterogenous hardware and must migrate VMs across that heterogenous hardware can't make any such claims.