First, be sure that you are not comparing the price of owning streaming rights for a movie vs. owning the actual movie file. A lower-bandwidth version of a movie that you may not always have access to and also have to pay for the bandwidth to watch is a lower value than a high-bandwidth file that you can watch anytime, with or without network connectivity.
The price for a disc of a given movie is typically comparable to the same quality-version of the movie on Kaleidescape’s store, for the titles a disc retailer has available. An issue with physical discs is the problem of inventory management. If supply exceeds demand, the retailer may work with the content owner to receive pricing concessions in order to move the inventory. There is never an inventory issue with digital products that would cause a content owner to be willing to make price concessions.