They look comfortable because they are soft, lightweight, and easy to slip on. The roomy design also makes them feel relaxed for indoor wear.
I would usually wear them at home, especially when doing everyday chores or just relaxing after a long day.
If I had to stay barefoot all day, I might not be fully comfortable because my feet like a little protection and warmth. Barefoot can feel nice for a while, but slippers are better for comfort and keeping my feet clean.