So, if this is such an obvious thing, why wouldn’t you think there are countermeasures for that? As people have said, the trucks have (or very likely have) axel

So, if this is such an obvious thing, why wouldn’t you think there are countermeasures for that? As people have said, the trucks have (or very likely have) axels that self-destruct. They also could have a trailer hitch that self-destructs if you attempt to disconnect it. The hitch could also be custom made to be too large/small/wrong shape for a traditional hitch. The lines that connect to the air brakes (that lock up without air pressure) could be custom made to not fit, or self-destruct, or both. The cab could have a kill switch that destroys or disables the engine. And of course, we don’t actually know which of these are actually used, and what additional security measures there are. So any bad actor would have to gain access to detailed security plans without it being discovered or attack blind and hope they have accounted for everything and can overcome known and unknown security. Seems a bit more complicated than just unhooking the trailer and driving off.