Why? There aren't very many obstacles at altitude. Humans routinely travel at 40mph+ too (bicycles, motorcycles, cars) and have to contend with much more diverse and surprising conditions on the ground. (Obviously, airplanes go even faster, but similarly benefit from a lack of obstacles.)

From an engineering point of view their control loop must be much faster. Things that fly need much tighter loops than we do that lumber about on the ground.

Watch a bird come in fast and then land on a narrow twig.

Bird neurons are roughly half the length of mammal neurons. This means you can pack in around 8x more neurons in a given volume.

Yeah, you can just (try to) watch them move their heads around looking at things. They move faster than the human eye.