The anatomy was named before they understood the evolutionary relationships between living reptiles and birds and extinct non-avian dinosaur lineages. There were dinosaurs unrelated to those which evolved into birds which had hip anatomy which looked similar to the hips of modern, living birds, so they described them as the group with 'bird-like hips,' not knowing how confusing a hundred more years of paleontology research would cause the name to become.

So, birds have bird hips, not lizard hips, in a purely descriptive sense that ignores established terminology, but these 'bird hips' are not hips of the type that put them in that named and defined group of extinct dinosaurs. Those modern bird-like bird hips happen to be descended from the hips of dinosaurs which had hips that didn't look as much like modern birds' hips at the same time as there were dinosaurs which, coincidentally, happened to have hips grossly like modern birds', despite not being direct ancestors of birds.

Coincidence, lack of current knowledge when anatomy was named, and the classic problem of taking descriptive morphological terms to imply origins, which is really the terminology's fault. Is this hip business really any more confusing than that we use the 'saur' affix which means 'lizard,' literally, for non-lizard species? Lizards are a specific thing, and dinosaurs (pardon the term) aren't lizards....but nobody's complaining about that term, are they? Because they were named when nobody knew any better, and we all acknowledge and agree that the name means they're like lizards, but real big and scary-like, to keep the term useful, not forget the literal etymology, and finally not ignore the newer science.