Motorcycle helmet is fixed to your head whereas a hard hat is not. In many situations an impact sends the hard hat flying with minimal kinetic energy imparted

Motorcycle helmet is fixed to your head whereas a hard hat is not. In many situations an impact sends the hard hat flying with minimal kinetic energy imparted on the former occupant. In those cases a motorcycle helmet would be worse because the wearer's head would move around more making neck injury more likely. For top of the head impacts they're likely close to equal since the strap system in a hard hat is pretty damn optimized for that use case. For repeated impact (fairly rare on the job site vs the default in motorcycle crashes) the motorcycle helmet is obviously superior and it has superior coverage. This is kind of an apples to oranges comparison. Wearing a motorcycle helmet (or any helmet that covers the ears) massively reduces situational awareness and wearing one 8+hr/day for the length of one's career would likely not be good for ones neck. It is not the right tool for the hard hat's job (and vise versa). Disclaimer: Not a helmet engineer.