That's part of the problem. And humans do constantly get horses to do things that they would rather not do. Hence all the riding crops. bits, saddles, training, martingales, reins, spurs etc. Do you honestly think that's how those horses would choose to spend their time, if they had the choice?
Just in case anyone who is non-horsey is reading this ^^ and is concerned, can I just say that if you are using a crop, a spur, or leg and hand aids, or tack, to force a horse to do something it doesn't want to do nowadays, then you are doing it all wrong!
Apart from the basic fact that it's hard to get a 300-500kg animal to do something it doesn't want to do, a crop should be used as an extension of your arm, to steer, never to hit. A very, very light touch (not a jab) of a spur (which should only be worn by experienced riders) is often kinder than someone kicking a horse in the flanks (if you are riding properly you should be able to ask your horse to go forward by a slight shift of weight and a squeeze of the seat bones, it's not necessary to kick).
And standing martingales or other tack should never be used to correct a horse's position or head carriage. This is done very gradually over time to build up impulsion from behind and allow the horse to carry itself and the rider properly, relaxed and in a good outline. The rider in turn should have a balanced, independent seat.
Of course a horse should be trained, just as you would get a child used to its nursery, you gently start to acclimatise a horse to people, being handled, cars, other horses and dogs, tractors, different environments, etc and the more calmly and gently and patiently you do that, the less bothered the horse is by standard situations which is good and safe for the horse and it's rider. And nowadays a lot of training is done on the ground through natural horsemanship methods. It's considered really bad form nowadays to put a horse in a situation that it hasn't been prepared or trained for.
I was lucky enough to watch a display of police horses at Buckingham Palace mews one year and it's amazing the training they were given and what they could do! Training, if done properly is a very good thing because it enables a horse to be calm and relaxed and completely untroubled in situations which could otherwise be scary. And btw, horses aren't necessarily scared by the same thing you and I are scared by. They could leap over a massive ditch without turning a hair and then shy at a tiny puddle!
The emphasis now is all about communicating with your horse and eliminating all signs of stress. A horse cannot learn if it's scared anyway. And you work very slowly and patiently with your horse, listening to it, so you can overcome any tension that it may hold in its jaw, neck or back. Only then do you take the next step in training. The welfare of the horse as an individual should be at the very centre of its education.