An online WDW Dining reservation system is probably already in some phase of development.
Companies such as airlines and hotel chains love online reservation systems because of significant cost avoidance. Conventional call center operations are labor-intensive, and thus expensive to operate. In addition, there are space and telecom costs (although Disney's telecom costs must be fairly low because most guests call 407-WDW-DINE, with the guests paying the long distance charges).
In fact, some airlines now charge more if you book with a human agent rather than at the airline website.
The problem is that WDW Dining would require a highly customized system. It's not a simple matter of loading a standard online shopping system onto a big server and being done with it. The online system would have to interface with the legacy dining system, and such interfaces are always a challenge.
An online WDW Dining reservation system would cost millions to develop, implement, and operate. But it would be likely to save even more millions in the long run. Also, if developed right, the reservation system would be integrated with a user interface that allows guests to learn about their dining options, view menus, view video about each restaurant, and, in general, and be "sold" the table service dining options. Remember, most guests don't have the knowledge and experience that DISboards participants have.