It’s an ocean-going row boat with 2 cabins. Most row boats you’ve seen are probably hyper-light and designed for still water.

The model of this boat:

https://www.rannochadventure.com/boats-2/r25

I always assumed it was because they had storm resistant cabins for sleeping and supply storage.

Daily use boats probably don’t need as much in that respect.

It's about a compromise. You need enough space for all the food and whatever other supplies you need on the way. You want enough stability that it's not going to capsize in whatever waves. Remember that a storm could come up. Her boat has solar panels and a navigation system and other things which are certainly nice to have but not required. I would expect she has some sort of radio on board.

The Polynesians appeared to have used basically canoes with an outrigger to row across the ocean. I'm not an expert on Polynesians, though, and how they got across the ocean. So if someone is an expert, please correct me.

Because there are big waves in the open ocean and people need to sleep?