Why is a transit visa not given to cruise ship passengers who visit usually for less than 10 hours.?

When will you leave China, by the same cruise?
If so and you leave within 24 hours, the Transit G is not needed since you only stay in China for less than 10 hours. If you want to leave the cruise for a short time sightseeing, you just need to apply for a stay permit at the port to leave the Port.