Because it's not about how expensive it is to actually make the drug. To use a software analogy, Oracle doesn't price their database software by what it costs in bandwidth for you to come to their site and download the latest version. Or in the days of shrink-wrapped software, the cost of software was far from the cost of the materials (cardboard and discs).

Books similarly aren't priced based on the cost of printing and distribution.

The cost is directly related to recouping R&D costs as well as the clinical trials. In addition, with these therapies specifically, you actually have pay lots of highly trained people to administer them in the first place.