I used to think that too... But here is the math...
I just booked myself on a 7 Day cruise in December for my wife and I.
From San Juan, Puerto Rico visiting Charlotte Amalie, St. Thomas, U.S.V.I. >> Bridgetown, Barbados >> Castries, St. Lucia >> Basseterre, St. Kitts >> Philipsburg, St. Maarten
We have a room with a balcony. Cost is right about $200 a day for the two of us. Any decent hotel will cost you about $150 a day minimum. But the ship includes food and entertainment for that price. So it really is cheaper to cruise. You can get cheaper rooms on the ship too. (ie no balcony)
Oh, and about the recent issues with ships. You have to remember, there are literally thousands of cruises every year. A small handful have had significant issues the last several years.
