First question to ask is whether we want realism or genre emulation. Everyone wants horses to travel in fantasy games. If we want the players to gravitate to horses in game, there must be an advantage to travel using horses.
To me, the advantage isn't speed of travel, but carrying capacity. The horses can haul a lot of baggage without slowing down the pace of travel. I'm pretty sure that was stated to be the case in at least one older edition, but probably got removed for the easier/more intuitive "movement speed dictates speed of overland travel" model in later editions.
Man in the Funny Hat nails it, though. "And maybe the DM is just tired of rolling random encounters and announces that you simply get where you're going without any more fuss. And of course, if a merchant caravan of 10 wagons leaves city A to go sell wares in city B then it can't be THAT hard to get through, can it?"
Is it adding anything to the game to meticulously work out overland travel speeds, or to play through an endless series of navigation rolls, cam setup and breakdown descriptions, animal handling and veterinary horse rolls. random wandering monster checks, etc. etc. etc., or should we just assess some travel speed we can live with, announce that "it takes three weeks of difficult, dangerous travel through the orc-infested broken terrain so you are tired, dusty and sweaty when, at long last, you crest a hill and see your destination in the distance."
I haven't played in many games where the GM follows that with "Unfortunately, another band of adventurers beat you hear and cleaned out the dungeon before you arrived. Where do you want to look for new adventure hooks?" Somehow, no matter how long and arduous, or short and convenient, the journey, the PCs seem to arrive right around the time that would best suit the adventure.