The publishers will tell you that it is because it is a lot of work to produce new editions every couple of years to keep the textbook up to date.

I'm pretty sure they are lying, at least as far as that explaining the high prices. Here is why I believe this:

http://www.amazon.com/Calculus-Vol-One-Variable-Introduction...

http://www.amazon.com/gp/product/0471000078

These are Apostol's excellent two volume undergraduate calculus textbook. Volume I is $231.25, and volume II is $189.49.

They were around $22 each when I bought my copies in 1977. I'm sure that NONE of the subsequent price increase is due to revisions and updates, because there have been no revisions or updates. Volume I is still the second edition (from 1967) and volume II is also still the second edition (from 1969). (There has been no need to revise or update them).

Based on inflation, these books should be about $80-90 per volume now (if we assume that $22 in 1977 was a reasonable price).