When a bird is perching or walking on the ground, it is almost never standing with its legs under the entirety of its body. A bird is like a rectangular box whose long axis is parallel to the ground with two sticks just slightly aft. In contrast, a standing human has a rectangular torso whose long axis is at 90 degrees to the ground with the legs coming out of one end.

To see this, do an image search for an Ostrich, which shows the 'box' effect perfectly. Smaller birds have a body box that is often at more of an angle to the ground, and with legs that are tiny compared with a human's. The bird's hips are near the back of the box but there are usually bones behind them, which is where the tail is mounted. The equivalent of the femur is tilted forward and short compared with a human's, and a bird's sternum may actually be between and below the equivalent of our knees.

Very different anatomy.

Birds are generaly balanced with their legs in the middle of thier body. Those with legs to the back don't walk as well (but that leg position gives other advantages that the bird might need)

Random uninformed idea… this is surely wrong but… birds ‘knee’ joints are the opposite of humans, have their ‘butt’ muscles have evolved alongside their wing/breast muscles?

Are birds mammals?

Birds are bipedal, but their trunks/spine are closer to perpendicular to their legs (while ours are parallel and stacked). Birds also have a non trivial amount of mass hanging down from their trunk between their legs, and their hips tend to extend significantly past their legs.

So while birds actually need significant posterior chain muscles to keep the femur extended, those muscles get to attach to an extended hip and so form more of a triangular/trapezoid shape compared to human butts. If you ever noticed that chicken thighs from the store seem a little triangular, and the bones are 'off center', then the the larger triangularish half of the chicken thigh is the chicken analogy to the glut complex.

EDIT: I think people would generally define the depth of the human intergluteal cleft ("butt crack") as one of the defining features of the human butt. In that context, I think humans have this feature at least partly because the human hip is... suboptimal in many ways. When considering the gluts as a hip/femur extensor, you have some pretty bad leverage, and so you're already biased towards having a very bulgey muscle there. As opposed to something like our hamstrings which has a much better leverage scenario, and so you see much less of an obvious bulge. By analogy, since birds tend to have an extended hip to attach to, their glut max analogs are no where as bulgey.

Birds are small, that means you don't need much muscle.

Birds are not mammals.