Meant to respond to this sooner
It might not seem fair to use a similar formula for GO users when you consider that ridership revenue is only used towards operating costs and not the capital costs, the reality is that GO is in fact competing against vehicle usage. And people make a decision to use transit(aside from convince factors) based on a comparison of what it would cost compared to alternative methods of travel. And the daily costs of getting to work by car it will always be much more expensive to on a per km basis the closer you live to work. Regardless of where you are traveling from parking costs are the same for everyone. These fee's account for a significant portion of cost of owning and driving a vehicle to work. The ratio gets even worse when you factor in all ownership costs(vehicle cost, insurance, licensing, maintenance). While living farther away costs more overall, it is much cheaper on a per km basis for car ownership.
Aside from that GO's main function, at this time, is to reduce congestion caused from those traveling longer distances and not short distance users. Short distance users afterall have more viable transit options available to them. As the system evolves(RER, electrification) it will eventually be better able to handle increased usage by short distance travelers and presumably/hopefully, the fare formula will change to promote such use when the time comes. But at this time it is not ideal for GO to promote such usage as the system is not designed or priced to be convenient for short distance users because it would overwhelm current capacity limits.