That's probably because you either have a Hollywood version of archers in mind where every arrow kills a guy. In real life archers hardly ever got a lot of kills, their role was to disrupt and harass.
Or it's because you're just equating the role of archers and javelin-throwers because they're both ranged. Throwing javelins mostly served to destroy/disrupt an enemy formation before the charge. A very different dynamic from the rather slow, withering fire of archers.
For example during Antiquity, skirmishers using javelins/pila/fransisca were probably a lot more common than archers who, save for the notable exceptions, were about the lowest class of soldiers around.
The Orcs being so melee-oriented would actually fair well by using javelins.
It's definitely not the Hollywood one. I'm the kind of jerk that likes to try to pick out the inaccuracies in movies and such, especially when it comes to weaponry.
But yeah, I'm thinking too much in terms of game mechanics and gameplay balance than actual use. Like, if one side has archers, the other needs something comparable for balance. Like you said, javelins and bows aren't really comparable. As far as balance for a game goes, you'd probably have enough wiggle-room to make an Orc Javelin-thrower fairly comparable to, say, a Breton longbowman. As far as reality goes, I definitely agree that Orcs would be more suited for using javelins tactics with maybe a small contingent of archers.
I don't really know the effective range of javelin-type weapons, but I'm going to guess that a bow would be more suited for sieges where you'd need to get a barrage up over fortifications to harass the defenders. Whereas a javelin would be for more direct combat, or to defend those little Orcish strongholds from a body of troops. Again, I don't really know how far a javelin will go—especially if you put Orc muscle behind it—so I could be wrong.