Not sure I see the problem?
It is very detailed, but isn't that the point of a spec? The alternative is getting/accepting a low bid where the vendor puts in a homeopathic dose of vanilla and one lousy cherry, resulting big profits for them but a deliverable that's almost entirely but not quite, unlike fruitcake.
You could imagine a different system, where somebody tastes existing fruitcakes from different bakers, then picks one that seems like a good value for the money. However, there is still the problem of ensuring that the delivered product matches the sample.
Moreover, it could, in theory) allow for corruption--how do you know the taster and baker aren't colluding? The US (and especially DoD) are set up to avoid that sort of "individual corruption", at the expense of much more bureaucracy. This isn't totally successful--somebody will instead lobby to reduce the maximum cherry size to 1/3 inch to exclude a competitor--but at least it's an ethos.