> If there's no crew on board then there's no risk to human life or ransom to pay, so the consequences of a pirate attack are lessened.

I don't think this is really true. Each container is worth $20k - $1M,

https://people.hofstra.edu/geotrans/eng/ch3en/conc3en/table_...

so the whole ship of ~10k containers

https://en.wikipedia.org/wiki/Container_ship

would be worth $200M - $1B or more. With 20 or less crew on board, and lives being valued even in the (expensive) US at only ~$10M, it's very likely that the value of the cargo is much higher than the crew. Not to mention the cost of the boat itself, which can exceed $100M.

Those numbers seem to be confirmed by this talk

http://static1.1.sqspcdn.com/static/f/1340097/17625392/13341...