Birds (especially large birds) inherited a long line of large, fast-moving, bipedal predator ancestors. We don't really have that; primates evolved ~55Mya, and

Birds (especially large birds) inherited a long line of large, fast-moving, bipedal predator ancestors. We don't really have that; primates evolved ~55Mya, and before that we were mostly rodent-ish animals which depended on being small and hidden to survive rather than running fast or long distances. There's a lot to be said for parallel/convergent evolution, but it's certainly not hard & fast rule.