People have been breeding horses for the different disciplines : their conformation makes their physical abilities.
Some are meant to run fast, some to pull carts, some to do dressage.
Breeding for work ethic (fast learning horses) can also be done.
Like western horses who are usually selected carefully for their quiet disposition and trainability.
Having (or not) the capacity of sitting has nothing to do with the capacity to learn the work.
Like me with running. I can run fast and I have a good cardio but no matter how eager I am to learn and train,
I physically doesn’t have what it takes to be the next Usain Bolt. (Even if I would stuff myself with all the enhancing drugs available on the market)
Like a downhill horse doesn’t have a learning disability. It’s a conformational one.
You just cannot mix a physical and a psychological aspect.