It was a reasonable assumption to assume that selling books online was unviable. It was enevitable that the cost, speed and quantity of data online would exceed all printed books. However, quality and conciseness hasn't been exceeded in the general case. (Encyclopedias are a debatable exception.)

The reason may be a case of game theory. If I cannot immediately find the information online then my choices are to search harder, find an expert, derive the answer myself or buy a book. Of course, when anyone takes the latter option, it discourages said information from becoming freely available in the near future in its entirity.