That’s only the case for cargos that are volume limited, not weight limited. That’s why Frito-Lay is an early adopter. For most freight, the extra weight of the batteries reduces the legal amount of cargo they can carry. The loss of carrying capacity costs a lot more than the fuel savings for long haul trucking.
Local delivery trucks should be the first ones to electrify. They have a lot more stop and go and have fairly consistent routes.