That's usually because the architects don't give a shit about the storage model and recognise that the external interface/contract is far more important and that can be mapped to whatever schema supports the model. That's their domain. All they see is: "how can we ensure that this whole thing still works when someone does scenario X".

The schema is almost 100% irrelevant to us and we have nearly 2000 tables in our system. We have two hibernate implementations which put that all behind a testable interface: something SQL can't give you on it's own.