Because the value of the envelope lies in the package that it delivers. It liberates us from delivery concerns and focuses our thoughts on the package. And it forces us to recognize that the envelope can have different meanings for different packages.
We need to leave some wiggle room for packages to interpret the standard flexibly. And while it would be nice to nail down the guid, we don’t really want to shackle all possible packages with our existing weblog/aggregator implemenation baggage.
So please distinguish between what the standard wants, and how application domains interpret.