I'd say the reason for the behaviour is that the wizard does not have to relate the source to an existing table - it is just going to create from scratch using your selections. In order to relate the source to an existing table it would need to be able to load that table (right down to its definition and not just the data in it) and allow you to say what goes where. Seems like a reasonable limitation to me.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.