Wondered the very same thing!
Military versions are off-road only and are illegal for road usage due to having no proper equipment (DOT approved turn signals, mirrors, low-high beams etc) to meet U.S federal regulations.
Conversions do exist; no doubt; but they're a pain to register from The DMV (Department of Road Vehicles) and cost more than its worth (At least in The U.S).
@tniazi Would love to know how you managed to imported TEN military Hummers to Germany from The U.S Army surplus auction and then got them approved and licensed. Gonna be an interesting read!
Most importantly; The V8s in Hummers are HEAVY polluters and I've no idea how they were able to pass EU's extremely tight emission regulations!
I'm sensing a fake story here but... I've been wrong before so who knows?
Good Day!