Because historically there were many forms of slavery and the culture of slavery in different places and times can be very different from what your probably imagining.

Chattel slavery - the ownership of a person as private property - is what many Americans are familiar with when 'slavery is used as a term. It's how slavery in American history was structured and organized. Ancient Israel did have chattel slaves but these were usually foreigners for them and it was less common than debt slavery - someone selling themselves into slavery to pay off a debt. It was also not an uncommon form of punishment for theft or property destruction.

This was not a permanent state and there were strict rules about how you could and couldn't treat a debt slave (because the expectation was that they'd be freed eventually). Jews were explicitly expected to be freed after seven years (Exodus 21:2). Slaves in this model still maintained many rights under the law and were not personal property. Basically, the master owned the slave's labor but not their person. It was possible for a debt slave to become chattel but I'm unclear of what this entailed.

Scholars of the Biblical Criticism model have proposed slavery practices were quite controversial among the Ancient Israelis given how frequently their books talk about it and how slaves should be treated. Numerous OT books criticize the treatment of slaves and practices of enslaving people (especially other Jews) against their will. It was not for example unheard of for people to be kidnapped and sold into slavery and you'll see this habit denounced and called sinful in Exodus, Deuteronomy, Jeremiah, and I believe Leviticus.

Indentured servitude, very similar to debt slavery, was quite common in early Colonial America. It's like debt slavery but you sell your service in exchange for a sum or in the case of people coming to the Americas passage to the New World. This was basically contracted labor. I want to go to America so I agree to work on your farm for 5 years if you pay for my passage and house me during that time. It was a way for land holders in the colonies to get labor from England at the time. As many as two thirds of the colonists in the 17th century arrived as indentured servants.

It receded heavily as a practice in the 18th century and in 1833 Congress banned the use of slavery as a punishment for debtors. That didn't end indentured servitude per se, but a runaway indentured servant was a debtor so the practice basically died as a result. It had been in steep decline since the Revolution.