Horses are significantly more valuable being a work animals so that makes sense since historic damages from loss of a horse would be severe to potentially lethal for some. While bikes can be important for couriers a rider can only carry so much before starting to impede their own ability to travel. Meanwhile with the right kind of horse could pull a cart with magnitudes more weight in goods, sometimes things which were the sole livelihood of a farmer, across ground that bikes can't really travel well.