Let's imagine for a moment that we're the top creatures on the planet (that's not hard) and we're large and because of that life is pretty easy. We have to eat all the time because of our bulk, but food is plentiful and no one much bothers us, and we live a placid, easygoing life. Would you trade the security for something new like being able to fly? Flying would mean you would have to be a lot smaller, have more fragile bones, and you'd be more vulnerable (except you'd have a great new way to escape a predator). And you'd still have to eat all the time, because of the huge energy requirements of flying and feeding the furnace of a power plant that your heart would have to be.
That's more or less what dinosaurs chose over hundreds of generations. Or nature, rather, chose those individuals that moved towards flight, even if that meant coming in a smaller package.
To me the great fascination is to look at birds and realize they are what's left of the dinosaurs, and to wonder what of their habits and social order is still like those ancient ancestors.
And then I consider how many times evolution has favored flight, and in completely unrelated lines of animals. Insects, reptiles (into birds), mammals (like bats), even some fish that can fly for short distances (and who knows where that will lead in millions of years).
And then there are birds that evolved back out of flight? Kiwis, moas, ostriches, emus, rheas, domestic turkeys. OK, I guess that last one is clearly a case of anti-evolution, or an example of how evolution changes when nature is doing the selecting through human values.
And if everything is trade-offs of some sort, what are we giving up and what are we getting right now as we change all the rules with memes taking precedence over genes? What more ancient bulk might we be shedding, and for what new sort of flight?