Weapons kill people. This is not a moral statement, but an argument in favour of one hit systems, with possibly two hits for the toughest of the tough. These are an excellent choice for brutal LRPs (eg Zombie), but not for a heroic LRP. Once you get past the idea that a normal blow should have an unarmored unfortunate on the ground, you also have to step past this same idea for other weapons, which includes the javelin. Any arguments that cite immersion, reality and climax are void once you have multiple hits.
Which brings us on to “why would anyone chose thrown”, and that is a simple case of how effectively it combines with other strategies. Every weapon choice in Empire has something that makes it cool, and the thrown weapon is no exception. The Roman javelin strategy of throw just before close would work on an Empire field.
Which puts us in a position of “why does the bow do impale” rather than “why don’t thrown weapons do impale”. A problem with most LRP systems is that the bow is underpowered compared with other options. It is two handed, like a spear, but doesn’t protect you. It can get one shot off in the time it takes your target to close, like thrown, but cannot be combined with a shield. There has to be something to bring archery back towards the coolness of other weapons choices, and in Empire this is the reward for picking your target or getting a good shot.
Also, referencing a history in which the bow and arrow have dominated over thrown ever since iron tools and skilled tradespeople (ie everything between Roman and Gunpowder) is a brave choice. I can point you at a large number of sources that point to archery being a much more effective weapon across several time periods and cultures. Which would you rather a picture of here; the Bayeux tapestry, Feudal Japanese silk prints, the sinking of the Mary Rose? The Mary Rose is a particularly good one here. In a potentially wet environment (which increases the fragility of a bow), after the advent of gunpowder and in the pride of a major nation they found vast quantities of longbows and no quantity of thrown weapons.