Depending on the components used, the price can add up rapidly.
Ignoring the WL Torch for the time being, and looking at a self assembled equivalent, you have the price of the Maglite, the glass lens and the metal reflector. Depending on the source of lens and the type of reflector used, price can vary quite widely on these parts. If you were looking to use the AW multi-level driver, that part is $75 on it's own. Let's not forget that if the suppliers of these items aren't on your door step, you have to include postage. OK, the WLT doesn't have the AW driver, but they still need a revised switch assembly with a socket for the bi-pin bulb. Bulb prices can also vary greatly with prices from as little as $4 to $15 - obviously pays to shop around, but you can see how (buying locally for instance) this could boost the price a bit. You then need a power source, which could be four IMR li-ion cells, 12 high drain NiMH in fivemega battery holder (12AA to 3D battery pack is a $38 item, not to mention the additional cost of boring out the Mag tube to take the battery holder in the first place, though that could be avoided with a 12AA to 4D adapter), or a custom assembled 12-13 cell battery pack (for which the price could be open ended). There is then the price of the charger for your power source (which WL include).
Basically, it all adds up.
Yes, a mass produced item should be cheaper than the above, and no doubt you could make one with budget parts, but for convenience, some people will be happy to be pay extra for the convenience of getting a fully functional light straight out of the box, even if it is missing some possible bells and whistles such as a multi mode driver.