I agree with all of the above posters However there is another factor.
A Lot or at least many of the sight to see in the world is away from ports where cruise ships frequent. Therefore in order to see those sights youmust travel on land to see.
We have taken many land tours and have visited many interesting places in the world. There is a lot of disadvantages in Land Tours or travel. Mostly is the packing and unpacking mostly on a daily basis. Bus riding can really get tiresome. But lets all face it you must take the hardships in order to see the world completely.
But we enjoy the cruising until we say get me on a ship and I will make the best of it. We love cruising, but we will still do land tours.