I can think of a few of the obvious reasons, I'm probably missing a bunch more...
1. The $168 computer is a set of commoditized bits and pieces, super low margin. Only one other competitor in the medium/high end groupo market exists, but due to (a) brand loyalty (campy vs. shimano), and (b) the fact that most bike manufacturers just use shimano for an abundance of reasons, there really isn't much competion.
2. Shimano can and will charge whatever the heck they think the majority of the groupo-/bike-purchasing market will spend. Regarless of what it costs to produce.
3. Shimano spends LOTS of $$$$$ on sponsorships, marketing, etc... the $168 computer is 'generic' in that sense.
4. To keep the components buying public coming back for more, there is ALWAYS an "next new thing." Perhaps better than the previous product, perhaps not (i.e. flightdeck-compatible shifters...valuable or not we pay for their design). That means design resources, re-tooling the manufacturing lines, managing a more complex supply chain, etc.
5. In Shimano's defense: take an Ultegra (or any other) STI shifter apart (read a few posts down on the STI headache post) -- lots of light-weight, strong parts n pieces manufactured to pretty close tolerances.