There is proprietary data in custom metadata fields in camera-written jpegs and raw files (and raw files are in any case usually proprietary formats). The meaning of those is usually not described publicly, and sometimes the precise meaning is not clear anyway.
Adobe could copy the proprietary fields when writing new versions of the files (often in different formats), although they might argue that they've no business writing data in someone else's proprietary format. In some cases that maker's data, originating from the raw file, would simply not apply (or not be relevant) when written to a jpeg or tif.
I quite agree that it's a pity that proprietary data is lost, but at the same time it's not entirely clear what to do with private-format data, especially when you don't know exactly what it means.