I would think the cost to develop just the dimensions is not that high. I think it's all gotten very confused with everyone having different definitions...What's it cost to develop a platform? A billion dollars? You'd think they'd be eager to get some more mileage out of that with more vehicles on the shared architecture.
Ideally they would all have unique dimensions to match whatever needs they have, and share parts and systems. From what I gather, that's where the future is.