I’d bet that evacuation at sea would be quite dangerous and by the time it would take to set up a process to do it safely, they could be in port and off loaded.
My wife asked the same question and I reminded her. We have been on quite a few cruise ships. I said look at the people. Now Carnival generally has a younger population but I would imagine that 20% of the guests use canes, walkers, and electric chairs. How do they get these people off the ship and onto another one. That would be extremely difficult to handle, even into the lifeboats and then to the other ship. Of course they do not need ladders to get on the other ship but they still have to dock at the tendering door(s) and in the open sea that can be rough.