I used to think this as well, but my family and I decided to try one out and we explored way more than we normally do on holiday - due to the fact that you're in a different port each day, you end up visiting way more different places than a traditional holiday, where everywhere you explore has to be centred around your hotel. Cruises especially allowed us to explore a lot of cities and towns, which we normally wouldn't have gone to due to the lack of "chill out" time on city break holidays.

Of course, this is my experience of cruises around Europe, where there's a lot of interesting ports clustered closely together. The experience is probably pretty different otherwise.