Not to say they are trying to replicate Django's admin, but rather to present a semi-automated way of surfacing CRUD actions on your Rails models.
To go all the way to a Django-like admin, a Rails plugin might have to do things like tie into authentication, which is not standardized in Rails, and either create some of its own models or require the addition of attributes on existing models (e.g. for things like modification history).
It may be a combination of possibilities:
1. Rails devs create a bare-bones / scaffolded admin as needed
2. Rails apps that really need CMS/admin functionality build something highly customized to their app and haven't seen the value in generalizing it for the community (or it isn't easy to do)
3. Developers that need out-of-the-box admin haven't found a barrier to entry that prevents them from using Django instead of Rails
I don't know the answer, but I think game2studios makes a great argument that the 2 frameworks arose out of different needs.