This may be my ignorance, but aren't most distributions [1] just an Arch / Fedora / Debian / whatever base with a desktop environment and a few opinionated choices (UI tweaks, installed applications, etc.)?

[1] I realise CachyOS makes some kernel modifications, but is that typical?

I believe the difference is between Omarchy simply having some default configuration for certain applications compared to CachyOS having a repository with a larger amount of packages which are being maintained by the CachyOS devs.