One of many things we like about cruises is that ahead of time you'll find groups of people preparing for the same trip, wanting to introduce themselves, meet up ahead of the trip, plan activities before and during the #cruise, etc. I don't know if you get that sort of thing for train trips or hotel breaks.
Sadly, the groups tend to be most active on Facebook and people arrange things like "pre-cruise drinks in Molly's Irish bar in Lima" and you think "An Irish bar in Peru? Really!?"