There are a lot of very good reasons why. The number of different city pairs for which fares must be stored would overwhelm our host systems. Handing fare inquiries from users unfamiliar with Amtrak's fare structure would require quite a lot of good programming. Amtrak sometimes offers promotional fares in certain markets. Special fares apply to special groups like senior citizens and persons attending a particular convention.
Fares depend on type of accomodations chosen.
The zoned fare plans like the Air-Rail Plan and All Aboard America have special rules. Fares can depend on the day and time of travel.