Sure, but now we're talking about game developers making choices based on distribution and patching instead of what they would likely view as more important considerations, such as performance and ease of development use.

Perhaps they could break stuff apart prior to shipping, but I imagine that could cause a lot of QA headaches as sure it should all work perfectly fine if a solution is used that transparently supports both combined and split formats, but who wants to bet the ship date of a multi-hundred million dollar project on that?