People get up to use the bathroom or the cafe car, the laptop is left behind for ten minutes, one of the train stops is while they're away from their seat, and someone sees an opportunity, snags it, and gets off at the stop.
This is an actual thing. And if it's worth a thousand bucks then it's very much worth getting off at an earlier stop then you'd planned, and continuing your journey on the next train.
Ticket inspectors or guards are irrelevant. There isn't one in your car 99% of the time.
I don't why you're trying to argue laptop theft on trains in first-world countries isn't a thing. It absolutely is.