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Thus in the case of the domestic chicken we observe a 2-lb forest bird having been bred into a 6 - 8 lb domestic bird which still has the wings for a 2-lb bird, and can fly just well enough to hop up into trees and over fences. Now, with all the billions of chickens around, with them not having been kept in fences or cages untill very recently and with all the billions of such which must have escaped and become feral over the millenia, if there was anything to evolution at all, you'd think that some small number of those would have retaken the air, that you'd look overhead, and there their progeny would all be.
I think you pretty much answered your own questions. I will try and help you along a bit. First of all, humans selected for size, not ability to fly. The ability to fly is still there but it is hampered by the increase in muscle mass, a three to four fold increase in overall weight. Just as an experiment, triple our quadruple your overall weight and see if you can still run.
For chickens to fly it would necessitate a reduction in size, something that farmers breed against. Secondly, there is not a selective pressure for flight among domesticated chickens. Given their present mass, it would take several morphological steps for them to fly free once more. In the wild each small step would mean be advantageous, but within their domesticated environment each mutation towards flight is not spread through the population through selective pressures.
Thirdly, the mutations necessary for flight would probably take a long time anyway, much longer than the time they have been domesticated. Recently, it has been shown that increased muscle mass can be the result of one mutation in the myostatin gene, so an increase in size within the time frame of domestication is not surprising. What would be surprising is seeing the wing from a 2 lb bird evolve into a wing able to support a larger bird in the same time period without selective pressures.
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