Really? No reason whatsoever?
I'm not being facetious, FWIW. I know a fair amount about PKI, in general (probably in more depth and intricate detail than the average HN'er, actually), but I'm not a developer and I know very little about code signing in particular (and even less when it comes to the Microsoft world of code signing).
I do find it kinda hard to believe that there's no use case whatsoever for this particular configuration (code signing w/o an included timestamp from a TTP), though. I certainly understand why a timestamp can be valuable (as it would in this case) but what isn't clear is that there is "no scenario" whatsoever where the lack of a timestamp might be acceptable or perhaps even desired.
As I said, though, I don't know enough about code signing specifically to know what these scenarios might be but I can't imagine there isn't even one of them.