Why didnt mammals ever evolve the ability to fly? Bats and flying squirrels dont count - they are odd. I mean real birdlike mammals. Flying with the eagles. Pecking with the pigeons.
After all they did evolve swimming like fish (dolphins and whales).
The answer, of course, is that they did. (Bats don't count? Whyever not?) But as the discussion progressed, the poster started reworking his question to ask why MOST mammals can't fly. He went on to point out the many advantages of flight (moving quicker to food, getting away from enemies, getting away from bad weather, etc) and concluded that there are no major down sides to flight. When someone pointed out that flying requires lighter bones and stronger muscles, and such additional adaptations, he asked why mammals don't just go ahead and evolve those things.
Of course no law prevents this from happening, as should be obvious by the fact that bats did evolve in exactly that way. But just because flight is good for some animals doesn't mean it's good for all animals. Not every feature which is helpful in some way should be assumed universally available. Every species has strengths and weaknesses, and the effect of evolution is that it sort of naturally "chooses" an area of specialization.
It's sort of like if you asked your dentist why he doesn't know how to do open heart surgery. You might say "Why couldn't you learn open heart surgery? Don't you think that would be a valuable skill for you to have?" No one would say that it's not a valuable skill, but based on his personal circumstances and choices, he's a dentist. He has a career in that. It wouldn't HURT to know about open heart surgery as well, but the payoff would not be good enough to justify the extra time and effort that goes into learning it.
Similarly, it's not bad for an animal to be able to fly. But it wouldn't be very useful for, say, a grizzly bear to evolve into a flying animal. Because moving in a direction that makes flight feasible would require certain features to change that would make it less good at being a grizzly bear.
While flight is a cool feature, so is being gigantic and strong. Cool features don't come for free; every cool feature you have requires higher intake of food (unless of course the cool feature is a highly efficient energy processing system). At some point, the set of features you have is already useful enough that adding one more cool feature is not useful enough to justify the energy cost. When that happens, your species doesn't get to keep the new mutations. Evolution works in small steps, and any trade-off which proves to be a bad one in the short term gets eliminated.