There's something to that, too. The ancestor of the turkey that ends up in the Roast Without Equal migrated from Europe to North America, where it developed into its modern form. Then it was brought back over by explorers, and raised and improved across the continent.
Every animal raised by humans is, in a sense, a living jigsaw puzzle assembled by global trade routes, market demands, technological innovations, and genetic accidents.
How we get a regular old turkey these days is much stranger than medieval chefs sewing together two different animals. For example, at any given moment, there are hundreds of millions of pounds of frozen turkey parts across the vast artificial cryosphere of refrigerated warehouses.
And not to make this point too strongly, but maybe these melanges, the cockentrices and turduckens, reflect something of the muddle of the human experience of the modern dynamic world. Turduckens just make sense to John Madden for some reason, no?
But still: why would anyone take the time to do anything like what we see in Tudor cookery, fantastical creatures aside? Another experimental food historian, James Materrer, has something of an explanation in his exploration of "incredible foods." He draws on German historian Helmut Birkhan, who himself drew on legendary anthropologist Claude Lévi-Strauss (emphasis added):
The imitations of dishes and special presentations assume a deeper, almost philosophical meaning if we consider the theory of Lévi-Strauss, according to which the preparation of food constitutes one of the major culture-creating achievements of humanity, the cooking of the raw an act of establishing culture. In these dishes man transcends nature either by transforming the foodstuffs (e.g. cooked peas turned into a hare-roast), or by preparing them in a nobler or more artistic form, as is the case with the special presentations. The chef thus becomes a creator, like the painter who adds symbolism to his depiction of nature, who transcends nature by capturing the meaning given to it by God through his creation of meaning.
What I see is that the cook's techniques could be seen as a metaphor for the power of the host. Think of it like the way the space program could be seen as a proxy for intercontinental ballistic missile skill. The spectacle reminds everyone of certain realities without requiring actual power to be exercised.
They did call the dishes subtleties after all (though the word had more the connotation of "clever" at the time).
Just as there is no one explanation for the bacon craze, I'm sure many motivations were at play for the chefs of the time: artistry, fun, competition, weirdness, technology, skill.
And most fundamentally, human beings are ridiculously, relentlessly creative. Given the resources, time, and context, we'll just do crazy things.
Why? Because we are human and we can.
How else can we explain El Bulli, Lady Gaga's meat dress, and cheese cultured from bacteria collected on humans?