Sure, you could get off the ship and flag a person with a sign announcing a route. But how are you going to have time to review review sites and make sure they're legitimate? By booking through the boat, you know that the tour has been vetted and, more importantly, supported. Luxury lines never make buses reach the gills, so even basic tours are comfortable and enjoyable, since you'll be traveling with a manageable number of people. Book a Ritzy Hotel Compared to staying at a hotel, comfortable luxury rentals with amenities such as a well-equipped kitchen and pool can reduce the cost of food, beverages, entertainment, and transportation, especially if you're traveling with a group.
Also, remember that luxury cruise ships carry far fewer passengers than mass-market mega-ships and simply don't have enough passengers on board to fill seats on a huge selection of shore excursions, while major lines have to offer a large number of excursions to minimize crowding at the most important places in the destinations. When many travelers want to take the same route, a luxury line will have several buses and guides so you don't have to worry about crowds.