Prepress operators are often on the phone with customers, helping them fix current problems and prevent future ones. Consumers turn to printers for all sorts of help with paper, file preparation, ink problems, scheduling, and mailing issues. You can't get this personal service over the Internet. In Boston alone there are over 800 print providers. Since printers have niches, corporate print buyers usually need multiple vendors.
Someone has to manage the vendor relationships.
All of this takes time and industry knowledge. Printing is highly technical, not something you can learn overnight. The new online technologies will only get better and better. These printing services will certainly appeal to people who are drawn to Internet shopping and who don't want to deal with the trouble of finding and managing printers themselves. Printing is complicated.