Compared to training infantry, who as you say only need the equipment and drilling to become competent enough to fight, training cavalry must have taken a good bit longer. Lets face it, as well as having to recruit them, equip them and train them, a cavalryman may have had to learn how to ride a horse!!

With infantry you are dealing with men who, even the most stupid of them, can be taught to stand side by side plus some more complex tactics. With cavalry you are not only having to teach the men the tactics but also the horses.