Why haven t mammals evolved strong large flying species?

We think in terms of "sufficient conditions" and "necessary conditions". I think it may be that mammals are not naturally good at flying, with short, slender limbs and tails, bloated torsos, and large heads,,, which are not at all dominant compared to birds or pterosaurs, they are light, have long limbs, and birds have broad and flexible tails, slender heads, and vestigial teeth. From the very beginning of evolution, the physiology and structure of the ancestors of mammals determined that mammals were not suitable for flight, and the sky itself was occupied by old flying experts birds, and it was difficult for mammals to compete with them from scratch.

Therefore, mammals are not well equipped to fly. And mammals do not have the necessary conditions to fly, because mammals do not have to survive by flying, insects are not the staple food of mammals, and the food suitable for mammals on land is more abundant, so there is no need to march into the sky. Even bats, the only mammal that can fly, have not evolved strong claws and large wings, but rather have a larger head and teeth.

Therefore, it is concluded that the ancestors of mammals began to have a strong trunk and head, weak limbs and tail, did not rely on the "sky" for food, and were more adapted to land and intellectual problems, and were not suitable for flying.