First up is space. Airplanes cram you into a small space like a sardine, with no room to even breathe unless you are in first class (which is not incrementally more expensive like on ships, but such an enormous leap in price that only the rich can really afford it.) In a car, you have more leg room, but are stuck in your seat with no hope for getting up unless you want to lose travel time. Train travel, especially certain trains, provide more space than the previous options, but again, sleepers are expensive and they still can't compare to cruise ships which provide ultimate space. You have your own room, plenty of places to walk around, sit, run, even swim. (Or float away in a hot tub :) )
Next, let's look at activities. Airplanes provide little entertainment other than some music you can plug into or maybe a movie on a long flight. My most recent flight did have individual screens for each seat with the ability to choose movies and television shows. This was a huge improvement, but is by no means an industry standard. Car rides are notoriously boring, but dvd players have certainly helped (especially with kids). Trains make it easy to talk to fellow passengers, but have few other onboard activity options. Cruise ship have ongoing entertainment with trivia, movies, shows, tastings, etc. If you are bored on a cruise ship, it is your fault because at the very least, you could be taking a nap in your large, comfortable bed.
How about comparing food? I think it is fair to say that, once again, this is no contest. Airline food is notoriously and rightfully known for being bad. Trains don't have anything to write home about, and cars have whatever you brought with you. Cruise ships have neverending food and, although it has its critics, the quantity (if sometimes not the quality) is hard to beat.
Of course I could go on, and I do realize that the cruise ship is an incredibly slow way to travel when compared to its counterparts, but wouldn't it be nice if all travel could be that easygoing and carefree? So, as we set out tomorrow for another long road trip, I'll be imagining myself lounging on the Lido Deck with a plate of cheese fries and the sea drifting by with maybe some great live music playing in the background.