To expand on the economic reasons: they are simply enticing you to spend more in one go, similar to super-sizing your portion in restaurants.
Especially when it comes to PRO lenses, like the Olympus you mentioned, or Canon's 24-105 that also comes in kit. Most amateurs actually crave the body more (camera body, that is), going for the latest and greatest, bells and whistles, while skimping on the lens, often settling for the cheapest (like 50/1.8 - though usually a great lens in itself). Or they go for other brands (e.g. Sigma, Tamron), that are usually cheaper than the original brand.
More experienced buyers know to get the best possible lens(es) they could afford at the time, and settle for a simpler body. The reason being, one, optical quality that impacts image quality more than a body does, and two, longevity of the investment. You see, camera lenses come and go every few months, while lenses last way, way longer. Just check the prices of Leica lenses, for instance, relative to bodies (other than collector items, of course). Especially when it comes to digital.
Other reasons being savings on packaging and handling. One instead of two SKUs (Stock Keeping Unit) means less warehouse space, less retail space, less bookkeeping, etc.