Because it's a Federal duck
There are collectors that buy the duck stamp for collecting as it is. Sounds like an idea to create additional revenues from the sales of the Federal duct stamp. Why not just hold a competition and create a Federal Birdwatching Stamp? Charge $25 a stamp, create a special division of agents checking "birders" if they possess the proper bird watching permits and stamps, and create the additional revenues the government wants?
What next? An Oriole flying over a pair of Blue Wing Teal for the Fed duck stamp?