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.

The first thing they'd have to do is either trick the convoy and its screening vehicles into a roadblock and assault the convoy, or use IEDs to stop the convoy and assault what's left. A number of people have commented on how the semi has its own countermeasures, but it honestly would take an awful lot of effort to even get to the point that such a thing is necessary.

I'm not sure you can just pull the hitch pin. The trailer itself is also designed to blow it's axles if required.