Okay, fair enough - but ponder this: assume, for the moment, that a fair number of overwater cargo shipments contain lithium batteries. Plenty of fancy electronic bits are packed into conex boxes and brought into ports every day.
Now ask yourself when you last heard of a major shipboard fire that was caused (or at least suspected of having been caused) by the cook-off of lithium batteries.
Then ask yourself why there might be a disparity between the two methods of shipment.