They aren’t looking hard at all. If you just had a super capacitor bank to capture regen, all the energy from the stop and go could be retained and they could probably run the truck on the batteries that they are currently saying last only 4 hours.
But "they aren't looking, why don't they just.." really rubs me the wrong way. Surely these people are experts in their own field and know what's available. After all, they are ordering trucks from a vendor, not designing trucks to their own unique requirements. Plus these Mack trucks indeed already have regenerative braking.
One only need look at the volumetric and mass energy density of diesel fuel and electric energy storage options to see that electric vehicles CURRENTLY store significantly less than diesel fuel tanks. That's well known and not a shock to anyone. Energy desnity of electric power storage (batteries, or even H2 fuel cells) will improve over time.