In Dragon Quest, you can make houses for your villagers. You don't need to do so and they are seen to survive well enough sleeping on the open but well, it's a game about building and building you will.

However, many players don't roof the houses they build because it makes it harder to collect hearts, add furniture or simply observe the villagers doing their things.

It's one of those kinds of things where gameplays necessities shape up aesthetics and if you played the game you totally understand why people do so.