It's a bit more difficult with a person on board.
Anyway, the human is a witness which adds problems.
He's also a potential murder victim, which drives up the potential legal consequences by a lot.
I don't have much real world experience with heist execution, so take my speculation for what it is.
Because an automated truck will not have cameras all around it and also inside, and will phone home whenever some alert has been triggered?
My understanding is the most common thing to do is take the trailer when the truck is parked and the driver is away.