This is easy to prevent. First of all, it’d be crammed with cameras. If it’s truly unmanned, there wouldn’t be a steering wheel, so it would be impossible for a human to drive, even if a robber could somehow gain access to the locked cabin. The trailer can also be secure with heavy locks. I suppose you could brute force your way into the cabin, but you’d need another truck to load the goods onto. So what’s the plan, drive an empty semi down the highway in the middle of the night, somehow hijack and break into an unmanned one unnoticed, load up pallets of dog food or onions or whatever the thing might be transporting, quickly enough before cops show up? Good luck!