why are lenses so expensive?
 

http://www.youtube.com/watch?v=X7_wL0ZZi6k

because they are a pain to make!

yes, basically.
Grinding the glass is very labor and time consumming. The higher the resolving power of the lens, the more grinding and polishing steps are needed. Hi-rez lenses have a much higher rejection rate, during manufacture, than low rez lenses.

And then there is the issue of AR coatings. The raw material is expensive and the process of coating is expensive. Baking in ovens for 10-24 hours is typical to apply coatings. QC is a manual operation, it isn't terribly automated. Handling is complex...clean rooms, etc.

All in all, it's a pain.