Good point. But think about it. Groups are security principals. They are the entity that is granted or denied access to a certain resource. A group is granted read access to a file. A group is denied access to the payroll file and so on.
Admin Units are the resources. They are closer in equivalence to the files in the above example. Sure you put users in them and they are a container in that respect, but they aren't security principals.
Then we get in to the age-old argument about "yeah, but seeing as they are containers (or more accurately, they are lists), they are a bit like groups, so why don't you just modify the system so that groups can be both resources and security principals. That way, there'd be no need to think about yet another container-type".
But of course that would require fundamental changes to the underlying system and the way it's been set out architecturally from first principles. Maybe we could have a special type of group that only has the properties of a resource, not of a security principal. Well, luckily - that's effectively what an Admin Unit is. But I get your point, on the surface it seems like inventing something that's already been invented. But hopefully this explains why it's not like that.