[KaiserPro]

AWS, and possibly GCP only allow 1:1 mapping of volumes (publicly, I know AWS allow it under the hood. )

Which makes synchronising snaphots a lot easier (and caching too, but thats another thing entirely.)

They are treated as block storage, so on the outside don't have to worry about what filesystem is running on it. (in practice they have to be a bit aware, so that they don't snapshot unbootable or dirty images, but I assume thats mostly handled by an OS plugin)

TL;DR:

AWS et al snapshots are at the block level. Linux has poorly documented primitives for this.

If you put your VM images on a Filesystem provided by ZFS or BTRFS then you can snapshot your images, without having to buy a SAN, or expensive controller.

ZFS has by far the best documentation. BTRFS's documentation has improved, but the tools are still difficult to use.