I had never thought about that before. Think of it. Birds came from dinosaurs, right? Jawed, snaggle toothed dinos. Where'd the jaws go? Why do ALL birds have beaks!?
Lightweight--it's made up of keratin (some thing as fingernails and the exoskeleton of bugs/lobsters/crabs and even the baleen in whales, etc.) which is far lighter than the dense calcium of bones and teeth. Isn't that something. Evolution is so purging of the superfluous that teeth got smaller and smaller until they finally disappeared and never came back, even for flightless birds that weight isn't as much of a factor.