Unlimited drivers are not possible with a flat rate, because we use Google maps enterprise plan. Google maps is not free. Per Google Google maps licensing
and Google maps terms
in Section 10.4(c)(iv), assets tracking needs Google Maps Enterprise plan. Google Maps Enterprise plan charges the fee per asset annually. So unless the other companies are not using Google maps, they can not offer unlimited driver for a flat rate.
Besides, there are many other costs associated with driver accounts such as SMS, push notification, infrastructure cost, support and development cost.