It would help program runtime as well because the macro could be operating on data not loaded in memory. Not having to marshal and un-marshal data can save lots

It would help program runtime as well because the macro could be operating on data not loaded in memory. Not having to marshal and un-marshal data can save lots of execution time, especially in the face of non-contiguous reads/writes. Databases store information using B-Tree's for example so that they can calculate the offset of the data and jump directly to it. It would probably take a lot of gymnastics to get this from an XML or JSON file interchange format. I'm sure they could probably come up with something that is both portable and performant but it's probably not a big priority at Microsoft.