We get it from scientific studies. It's a fact that birds need flight in order for their respiratory system to be utilized as Nature intended (posterior air sacs -zoology- and studies that show that muscle utilization is at less than 10% even when flapping and climbing -biology), it's a fact that when you deprive a bird of flight you are, in fact, taking away the only predator-avoidance mechanism in a prey animal with the unavoidable chronic stress this produces (basic ethology), it's a fact that baby animals that are not allowed to develop their motor skills suffer from a decrease in mental development (myriad studies on humans and other animals).
But, in truth, I did not clip even before I became aware of all these studies because, in my mind, handicapping an animal so severely just so we can keep it for our entertainment has always smack me big time of being self-centered and cruel (they used to do something similar to slaves that used to run away -it was called hobbling, same as what they do to horses).