Because you don't throw your reins up their necks and leave them to fend for themselves
A young dressage horse, fresh from the breakers so has no idea about collection, going into a 'frame' etc. is ridden very forward, motoring off their hind legs, into a very light, elastic contact on the bridle. The contact is essential for connecting the hind legs, to the back, to the bridle and to the riders hand.
It does not take long at all (in fact, most breakers under a good, unobstrusive rider, will be starting to stretch from the wither into the bridle within only a few rides after backing.
All we care about, is that the horse goes forward off the leg with active hind legs. He is ridden in this manner until he develops the srength to develop a little more carrying power behind, and can then begin to slightly shorten the frame. This is when the horse will start developing a good reaction to the rider's seat. The seat takes the job of the hand, in becoming the 'restraining aid' and can begin to influence the speed and carrying capactity of the hind legs.
In creating drive in the hind legs, with the 'driving aids' (riders leg), and containing this power through the seat to control the speed and carrying capactity of the hind leg, and completing the circuit with the hind, keeping a light contact with the horse's mouth - he will naturally begin to come into the typical 'frame' that you see in a dressage horse. The stronger he gets, the shorter and more uphill this 'frame' becomes. No need to pull on their mouth to pull the head down. The contact and restraining aid of the seat does the work, and saves the horse's mouth.
Joidigm, if you get the back end correct you don't need to worry about the front end as it naturaly falls into place. But that takes riding from your seat properly.
This is Reeco the first time I sat on him which was about the 4th time he had ever been sat on
He started as this
5 mins later we had this:
Then we went on a hack up the road and were offered this:
And this is him 9 weeks later, I have always ridden from my seat and he seeks out the lightest of contacts (like holding a budgie in your fist, got to be very very gentle). No fiddling with the reins, no taking a hold or anything just allowing him to work up unto my hands.
I hate horses who you have to hold onto thier mouths or fiddle with your reins to get anything, it is created by riders wiggling to reins untill the horse drops behind the contact to avoid it, or by riders widening thier hands and riding with thier hands on thier knees so that the horse has to drop its head to avoid the downward pressure on thier jaw. Its a **** pain to try and reschool as well because everytime you take a contact the horse will drop behind it.
I'd far rather see a horse above the bridle than behind it!