Why would someone want to go on a cruise ship, when he is not into food and entertainment. What else is there to do on a cruise ship?

Some do this just to relax.  

You can screw the other passengers.

Here's what I like about cruising- you can go with a group. During the day, everyone does their own thing- hang out by the pool, go to the casino, sit in a deck chair and read, sit in a sports bar, drink and watch sports, play games in the game room, etc. Then you get together at night for dinner. You don't have to spend every minute with the people you're traveling with. At night, you can go from bar, to piano bar, to dancing, to seeing a show- and carry your drink with you. At the end of the night, you don't have to drive home. You just stumble back to your room. Different cruise lines offer different  things to do. Carnival has trivia games, ice carving contests and other events, all the time on sea days. They also have lots of sports facilities- pools, slides, basketball courts, ping pong tables, and more. Holland America is an older crowd and has fewer activities. They do have cooking demonstrations and movies. Research the ships and the offerings on cruisecritic.com before you book a cruise. I have friends who love Holland America, so I've been on several cruises with them, and I find it very boring. I much prefer Carnival and Royal Caribbean. 

Gamble, hangout by the pool, shore side attractions, meet new people are some other possibilities