The answer is simple: unrealistic expectations. People buy a horse without thinking. They dream, but they don't think. They dream about riding the wind, riding into the sunset, winning at horse shows, flying over jumps, roping a cow, etc. The dreams are potent, lasting, intriguing draws to a romantic life of freedom and fun. Little to no thought is paid to what it really means to be around a horse, and a horse is 1,000 pounds of attitude.
It would be an ideal world if every horse glowed Hollywood-style. What a wonderful dream-come-true it would be if every horse knew, just as a saddle was strapped on and a bit put in its mouth, that he would carry you off into the wild blue yonder with nary a thought of balking, bucking, spooking, running off or running under the lowest tree branches around. Yes, the West was won from the back of a horse, but it all started way before the first saddle was ever thrown up on that horse's back.
Horses are intelligent animals and learn three times faster than humans. They know from the get-go that a person coming toward them with a halter and lead rope in hand means that their choices will soon become very limited, and so will their natural survival tendencies. A horse's primary means of defense is flight, and that halter and rope takes that away immediately. A horse tends to find and take the easiest path to comfort, and that it is far easier to cooperate with than fight the halter, lead rope and human holding the other end. And when that halter, lead rope and human lead to pleasurable things, it is no longer a threat and a horse will look forward to them. They will even forgive the occasional uncomfortable things like vet visits, farriers and wormers, saddles, bits and humans on their backs.
People are more intelligent than horses, even though they fail to notice that yes, a horse is learning. A horse learns something new each and every time a person is handling them. A horse lives in the moment instead of thinking ahead or getting lost in thought or planning the next move. Through repetition, a horse learns what to expect in the next moment, moment by moment.
Those big, beautiful eyes on the end of a long, flexible neck give a 360 degree view of everything all the time. When the horse's head is down while grazing, those eyes see everything happening around his feet, and when his head is up high, he can see everything around for miles. His brain is such that it processes input from both eyes as both give different views yet is able to switch to focus both eyes on one thing too. They see everything, always. The horse's ears point where each eye is looking as a backup, and the ears are constantly in motion.
Lost in thought, a person won't realize that a horse has already read and interpreted his intent through body language and the look on his face. A calm, confident demeanor is far more trustworthy than a nervous and jerky approach. The horse will tend to move away from that nervous, jerky behavior, or at the very least, will keep a wary eye on that person.
Body language and slight movements are a horse's first level of communication. Watch two horses together and you will see a slight, welcoming tilt to the head or a swish of the tail and a pinned ear in warning to stay away. If more warning becomes necessary, a rear end will shift towards the offender and if that's not enough, a back foot will raise. If that's not enough, the hoof kicks out a little, then a lot and then will hit and hit hard. It's war when both back feet kick together and the thud will be hard enough to knock the wind out. Sometimes, it will be the teeth, first a threat, then a nip, and then a full fledged bite. Unless horses are engaged in mutual grooming, that is about the extent of physical contact between two horses.
People, smart as they are, fail to understand the horse's means of communication and insist that a horse learns his instead without his first learning theirs. That 1,000 pounds of horse could kill a person in an instant, yet a person expects it to submit and obey their every demand. Since a horse soon figures out that people are none too smart after all, they learn to take advantage of the opportunities that present themselves. When the person fails to escalate the pressure and allows the horse to not respond to a cue, then the horse learns that the cue is meaningless and not a cue at all. The horse is not acting out or behaving badly, he is just doing what he is taught. Nothing. And, doing nothing is just fine.
This, of course, is just the tip of the iceberg. The moral of the story is that it is the person that needs to learn how to communicate with a horse in ways that a horse can understand, first, and long before saddling up and riding off into the sunset.