It kinda is. Canonical explictly doesn't want to do that. The cli doesn't support any other server except canonical's.[1]
Flatpack on the other hand, does support that structure out of the box, and a few projects are already using there own repos.[2]
[1] https://forum.snapcraft.io/t/external-repositories/176 [2] https://docs.flatpak.org/en/latest/repositories.html
It's not really an issue with snap itself.