There is bullet drop, but due to the high velocity of the bullets, it is almost not noticeable. Why cant you hit people that are far away, with a pistol? Simple. Pistols have much lower velocity and therefore their bullets actually drop much faster. Bullet drop is heavy on pistols, but almost not noticeable on Muskets (Except for very long ranges.)
Remember that when shooting a musket, the bullet will not go straight, but go randomly, into any direction. That is the reason why the aiming reticule is so large. When shooting, the bullet will go somewhere in that reticule.
The reason why you think that bullet drop is so heavy, is that a lot of times, the bullets do not go straight and do not go up, but downwards, decreasing your range alot. Aiming Up does not help here, because it can also be that the next shot goes up, shooting much further than you expected. Aiming anywhere close to the enemy can still hit him (Especially on long ranges, as the randomess plays a much larger role here), if you're lucky.
Therefore it is always best to aim straight at your enemy. Shooting with a Musket is just pure luck, nothing else.
Also, a little hint. Aiming 2 seconds with your musket will make aim steadier, and the bullet is more likely to be shooting into the direction of the center of the reticule.
Another hint... Warbands random function is not very good. Most of the time you will end up with the exact, or almost exact values.
So if you know that your bullet just flew low, try to aim abit higher. If you are a little bit lucky, Warbands random function might have chosen the same value again.
That is also the reason why you get the same bush/rock/tree over and over again, when making maps. Warbands random function selects the same value again and again and again.