Ports are too far away from the destinations and available taxis or other tour transportation is difficult to reach.
Review by ArtfullyAnns
In response to review, ArtfullyAnns says ...
I enjoy sea days, too. They give me a chance to relax. There is usually not too much to do right in the immediately vicinity of a port, I agree. At least in the Caribbean, more islands are adding shopping areas, bars and even pools. Wish all the ports had these amenities. Our local Port Canaveral added an Exploration Tower right at the port. There are also restaurants and a area called The Cove with vendor tents and booths. I usually take a shore excursion from the ship if I have never visited the port previously.