I have first hand experience with someone who chooses, not because they can't, to go the day of embarkation.
We had a family cruise planned and I had booked my family's flight 2 nights before, and another family member a night before the cruise. Another family member and her family decided that they have always gone the day off the cruise and they don't need to bother going the night before. I asked them why and they said, there's no need. They could go, but chose not to.
Well, of course, I went into hyper planning mode with "just in case" options. When I told them the options they can do if something does happen, they just laughed.
On the day of the cruise, their flight was delayed. I told them that if they didn't arrive by a certain time, the rest of us would go ahead to the port. 1hr before all aboard, she calls and says they're on their way. She was quite calm and I was on panic mode...
They boarded with smiles on their faces and didn't even worry. She then said to me...."see, no need to get there the night before"...needless to say, we have never cruised with them again.
I do agree that some people don't have a choice (work, schedule etc), but there are still others who don't think they would ever miss a boarding and so I do agree with disneymagicgirl as well, that some people just never listen to any warnings no matter how much you try to convince them.