Responses
Because slavery in one form or another has been and is a constant in history. Indentured servitude also came in a variety of flavors from basically an obligatory employment contract to more or less the chattel slavery everyone thinks of.
Yes indentured servitude can be really bad however manga often doesn't even show that. Much less just because it's common irl doesn't mean it needs to be in every one of these stories.
Okay, so I'm at chapter 15 rn, and the system of slavery they have in this world isn't really slavery it's actually indentured servitude. The main difference here is that indentured servants have contracts, are paid salaries, and are usually only servants for a few years or as stated in their contracts. Slaves have none of this and are legally considered property with no human rights; so calling this slavery gives a wrong impression of slavery.
Indentured servitude is still an awful system though since it's basically having more rights than a slave but less rights than a servant. Honestly, I think it was a completely unnecessary addition and I don't get why every isekai protagonist has to have someone in a much worse position of power serve them. It doesn't make them look good