The first reason is that birds are relatively small and light, and their fighting capacity is insufficient when facing some behemoths on land. We all know that no matter which animal wants to dominate the land, it must have the strength to dominate the top of the food chain and defeat other competitors. Birds have evolved the characteristics of petite and light weight because of their own flight needs. Although birds have air superiority against animals on land, their attack means are relatively simple, and their strength and lethality are very limited, which is not enough to subdue those behemoths on land.
The second reason is that birds need less food because of their small size, which determines that birds cannot always kill animals on land on a large scale, and naturally they cannot dominate the land. If birds want to dominate the land, they must always kill large animals on the land to improve their coffee places and make other animals feel scared when they see themselves. However, because birds are small and light, they don't need to eat regularly, and naturally they don't need to kill their prey all the time. Moreover, many birds have evolved to eat only vegetarian food such as plants and grass seeds, and will not kill animals on land at all.
The third reason is that birds have limited ability to resist the cold, and they need to migrate for the winter every winter, so they do not have the conditions to dominate one side. Although birds have feathers, their ability to resist low temperature is really limited. Many birds will stay away from the local area in winter and fly to the temperate zone for winter. This also determines that many birds can only stay in one place for a few months each year and then migrate away. Even if you can dominate an area, you will lose control of it after you leave, so you don't have the conditions to dominate the land.