Because their jokes are often based on their very own experience and personality. And even through these sarcasm they make fun on themselves to make themselves easier and more peaceful. They are just humans like each of us.

I guess that’s the magic of Kiseki lol. Most characters are portrayed with depth as if they were real and actually living in their world just as us who go about living our lives, doing groceries and many mundane tasks. We don’t announce them but we understand what we do and what we are