Why isn't it possible to pay free-flow tolls in the same way everywhere in France?

There is a payment solution that can be used on all motorways in France: the toll tag. Fixed to your vehicle's windscreen, the tag is detected when passing through the toll.
An invoice is sent to you every month and payment for your journeys is automatically debited from your bank account.
For other means of payment, each motorway company handles its own channels of payment.