Why do we still have electronic toll transponders?

Given that authorities can reliably send bills to owners of cars that fail to pay tolls by using OCR to read license plates, why do drivers still need to put transponders (e.g. FasTrak) on their windshields? The transponders seem unnecessary, especially when each one is directly linked to license plates.

On the Golden Gate Bridge, drivers can now opt to be invoiced for tolls without a transponder albeit at a higher price than with FasTrak. But, why does having a transponder lead to a discount, shouldn't it just be whether a vehicle has billing information on file, so that it's cheaper for authorities to collect?

Infrastructure changes always take time, but hopefully this transition will be quick. After all, this is more about deprecating old transponder receivers and fully relying on the existing camera systems than on installing new hardware.