As someone who has dealt with crazy tax codes and different regulatory fees in telecoms I can tell you that the systems that calculate those bills are very old

As someone who has dealt with crazy tax codes and different regulatory fees in telecoms I can tell you that the systems that calculate those bills are very old and clunky. They are built to handle billing in batches, and are not suitable to provide accurate amounts to be shown on the consumer facing website on demand. Of course they can build something that is suitable. But they are a combination of [lazy, greedy, corrupt, under-resourced].