Respectfully, then, i’m not sure if you appreciate how fine tuned a bird"s wing is alongside the other fine adaptations to a bird’s anatomy that optimize the flight involved, and just how very complex it would be to incorporate that structure into a mammal, both at the genetic and anatomical levels.
To ask it in a very obvious way… consider this: I could just as easily ask why not compose a flying mammal using wings that were readily available-i.e., from insects?
And let’s even assume we’re talking about the world’s largest insect and the world’s smallest mammal, so it isn’t just the absurd size difference. could we simply put fly wings of our largest insects on the body of our smallest mammals?
If I use that illustration it is perhaps more obvious on its face how prohibitively difficult this would be … insect wings work just fine on bugs, as their construction, aerodynamics, weight, shape, control, and everything else is fine tuned along with everything else about the insect’s anatomy, but would require a major overhaul of multiple other systems in your mammal for it to fly with insect wings. How would,you go about re-engineering the neural systems and muscles in your mammal in order to flap its new insect wings? how many new muscle groups would have to be invented and then integrated de novo, and connected to others, in order to make the wings flap? are mammal muscles even designed to be able to beat the insect wings at the rapidity that the fly wings do? would we then have to introduce novel muscles into our mammal in order to use the insect wings? If we did design muscles that could make the necessary movements for our imported fly wings, would they have the oxygen transport necessary to function? and new neural systems would also have to be imported and integrated into our mammal that has no preexisting neural systems to beat insect wings, and no instinct for how to use them. And how does our mammal even grow these wings, as we also have to develop all the new pathways for a mammal to create and transport the unique proteins, and how would you even begin to have insect wings growing out of a preexisting mammal’s anatomy?
I imagine it is hypothetically possible for the genes to make insect wings could be grafted into a mammal, and some ingenious genetic engineer might figure out all the layers upon layers of new integrations and adaptations you’d have to make to the mammal in order to get these wings to actually function and allow flight from our new organism. But i highly suspect that our hypothetical human geneticist would have far, far better luck trying to make adaptations to a mammal’s forelimb to turn it into a wing, and the other related adaptations. Complex though that would admittedly be, it would still be a far more simple and straightforward pathway than grafting insect wings into a mammal and the multitudinous adaptations required to integrate it. So it may not be obvious, but simply putting bird wings into a mammal would face similar prohibitive difficulties.