Just what the fuck is up with Japanese manga and slavery...
Best guess would be Cultural Baggage. From how deep-seated it seems to be in their media, debt slavery was very much a thing irl. Might still be in some form.
But honestly I don't even find this kind of slavery that bad compared to the "traditions" of some other cultures. I hear the Middle East has some real doozies that are openly practiced even today.
Have you ever heard of Female Genital Mutilation, its all the rage in Middle East, Africa, and parts of Asia. It involves partially or wholly removing the feel good parts of the vagina(because it's blasphemous for a woman to enjoy sex), rerouting the urethra(optional), and sewing the lips shut so that all that's left is a hole for conception and birthing.
I can understand that most forms of slavery is immoral. At its worst it robs a person of their dignity, autonomy, and freedom of speech. But the same can be true of many other cultural issues.
There are many parts of our world that treat half their populations little better then slaves. Few are allowed personal autonomy. They are forced to wear heavy clothing that obscures every part of their body, including there eyes. And many are legally killed for little more then aperceived offence. Yet it's not defined as slavery because it's seen a religious or cultural issue.
You might find what are for all intents and purposes debt-slaves in Japanese cultural history horrible, but m8, modern day Earth can be much much worse.