Just to get super abstract with the purpose of existence for a moment, the purpose of existence is this: To occupy time and space. In short, things which don't occupy time or space, aren't.
Here are some different ways to occupy time and space: Be big; be multiple; be long lived. Planets are big, so are stars. Grass is multiple. Rocks are long lived. So are trees.
One cheap way to occupy more time and space without being more materially robust is to avoid the termination of the occupation. Animals are smart and get out of the way of being terminated. Movement and intelligence are both good strategies.
Occupation isn't measured in metres or seconds but by witnesses. A dinosaur, being big, occupies space as far as it can be seen. To maximise its witnesses, it moves. There's a kind of persistence of existence which means it occupies a sausage shape in space as it walks along.
I have a theory that dinosaurs were silent, and they were replaced by birds because birds had a technological secret weapon that let them occupy more efficiently: Noise.
Birdsong occupies more space with less meat than dinosaurs.
If this is correct then, on average, birdsong in the late Mesozoic can be heard from a marginally greater distance from that which dinosaurs can be seen.