Some things are lists. Some lists can be joined to other lists. Our current generation of OODBMSs don't have the performance to join objects to objects as efficiently as RDBMSs can join lists to lists.
I suspect some of what's being marketed as OODBMSs are really RDBMSs under the covers with an abstraction layer on top for marketing purposes. Basically the same as what you have now with your home-brew object wrappers, except you don't get to go in and optimize the abstractions the way you can now.