Sab... a few thoughts in answer to your question.
1. Lets remember that the use of PC's for actual navigation duties aboard boats is low. Chart plotters and cartridges are far more common and since they are proprietary...there are no government free charts for this market. And so...chart cartridges can be sold for whatever the competition/free market allows.
2. Many people who plan to cruise outside of the USA have to pay for their charts. Example : Canada
3. Some people will pay for the RIGHT electronic charts for an area. Example...the Explorer charts in paper are the BEST charts for the Bahamas. Only 3 mfrs. are using them to produce electronic charts for the Bahamas. Avoiding running aground once pays for the software! Avoiding putting a hole in your boat pays for a lot more!
4. The proprietary charts now come with a lot of extras that people like. Harbor photos, NavAid identification,3D Views, marina info etc. that some are willing to pay extra for...sometimes just to impress their friends!