Many more people "sold" themselves into slavery to avoid starvation. Judgements in lawsuits were another source of enslavement as losers tended to not have sufficient property to pay a judgment with money or goods.
Slavery was also an alternative to execution for criminals. Societies were not rich enough to afford years of food& for convicts so misdemeanor theft got you hung, or impaled, or something. No, the Arabs modernized the slave trade 1200 years ago. Before that enslavement was mostly an alternative to immediate death.