It is a balance reason. . .essentially the system goes out of it's way to limit the defenses of people able to kill at range. . .
This makes it possible for a close combat character to survive into range to chop them.
The system is on the deadly end due to the crit system, so it's not a minor factor. As a comparrison, in AD&D one 10th level fighter with a Longsword facing ten 1st level fighters with loaded and aimed light crossbows is probably in a position where the smartest thing to do is charge, suck the damage, and hack them up before they can re-load. . .RM would make that a very ill advised choice.
At 1st level, 35 OBs, Armor makes a key element here. . .if the archers are in heavy armor, they will loose all OB and miss, if the target is in heavy armor, they may not take any criticals.
Flavor is why, but balance is why there's not "Melee OB penalty" (Despite the fact that logic would say that fencing in AT20 has to be harder than fencing in street clothes.). . .IMO the system is balance skewed against common sense to favor armored melee over missiles, to prevent the game from becoming one where all fights take place at long range. (Getting antagonists into melee range enhances drama, and allows for speaking. Sniping can be exciting, but it places distance from all encounters, which often leads to a mechanical/tactical game.)
Consider how boring a western would be if the hero killed everyone at 500' with a rifle. . .flavor and balance are hoplessly mixed up, because systems are balanced to provide a certain flavor. . ."Balance" in any complex system consists of a multitude of unbalanced elements that in aggregate balance each other. The choices you make in each individual element build to create the flavor of the game, but in the overall they are all balanced against each other. . .it's rather hard to discuss rules flavor without talking about balance, or vice versa, because in many ways they end up being the same things, just looked at in different ways.