There are a myriad of reasons.
It is an overall good value.
It is relaxing and entertaining.
It is great to just be at sea but also be able to see the world.
Overall: The food ain't bad either.
I got hooked on cruising around twelve years ago. After my third or fourth cruise I developed a problem in which there are times I am not able to travel and need to remain in one location or be able to get back to the ship or hotel in a relative short order.
Cruising allows me that. In the event I am not doing well one day I can just stay on the ship. I may miss something but that's something I've learned to live with. If I'm doing OK I can go and enjoy the ports and see places I wouldn't be able to otherwise.
It can be a detriment for me to do a group tour or cruise line excursion because I may need to get back to the ship and you can't turn the bus around for one person. It isn't fair to the others. This is why we usually book private tours when we travel.
Overall: I just love to cruise.