hawkeyefan
Legend
You get some dudes, dress them up in leather armor, and then whack at them until they fall down. On average, it will take fewer whacks from a longsword than it will from a scimitar, and while you're even more likely to drop someone with the first hit from a big two-hander, the fact that you need two hands to use it is enough of a reason that someone might prefer the longsword instead.
Or equip an army, and send them out to slaughter some orcs. History is full of that sort of thing happening, and the guys with heavier swords were more effective than the guys who tried to brute force with a lighter blade, after adjusting for outside factors.
Or set up a pile of wood or straw, and whack at it with various swords to see how deep they cut. All signs point in the same direction.
Or even easier, don't do any of those things, because it's immediately obvious to everyone with the tiniest bit of common sense that the bigger weapon has more stopping power!
You would be mistaken on that matter, within the game world which the rules reflect. Longswords perform better than scimitars when using a strength-based fighting style, whether you're mounted or on foot.
If your DM thinks the mounted combat rules are insufficient, and changes them so that scimitars actually are better from horseback, then your argument might hold ground. Until then, scouts will prefer scimitars primarily because they use a finesse-based fighting style, which favors the scimitar whether mounted or not.
I feel that you're kind of citing the real world when it suits you, and then citing the game world when it suits you.
In the real world, scimitars were designed to be used from horseback. So that real world fact is the basis for how the game handles that type of weapon. The fact that the game simplifies it so much is a necessity of forcing it all into a manageable scale.