It's balanced by the fact that you lose accuracy while you are moving on a horse. Also, you cannot use the top tier bows while mounted on a horse. It is fine, I think people just get frustrated if they are melee and there is a horse archer left on the field. They are supposed to be engaged by ranged primarily; if they are in range to shoot you, you are more capable of shooting them because you do not moving on a horse and a horse is a much bigger target. Horses being OP is overrated.
ALSO, it has always been this way, no point in changing it for any sense of realism, it has been fine , just because many people get frustrated because they can't engage a horse archer all the while being shot at, is no reason to QQQQQQQQQQQ. Remember, they had to drop points into horse archery and riding.