That's because slavery in that culture was not what we think of when we think of slavery today. There are numerous passages throughout the Old and New Testaments detailing how slaves were to be treated with the utmost respect, which was otherwise unheard of in antiquity. To give just one example:

If your brother, a Hebrew man or a Hebrew woman, is sold[a] to you, he shall serve you six years, and in the seventh year you shall let him go free from you. And when you let him go free from you, you shall not let him go empty-handed. You shall furnish him liberally out of your flock, out of your threshing floor, and out of your winepress. As the Lord your God has blessed you, you shall give to him. You shall remember that you were a slave in the land of Egypt, and the Lord your God redeemed you; therefore I command you this today. But if he says to you, ‘I will not go out from you,’ because he loves you and your household, since he is well-off with you, then you shall take an awl, and put it through his ear into the door, and he shall be your slave[b] forever. And to your female slave[c] you shall do the same. It shall not seem hard to you when you let him go free from you, for at half the cost of a hired worker he has served you six years. So the Lord your God will bless you in all that you do.

– Deuteronomy 15:12-18

[a]: Or sells himself

[b,c]: Or servent. The Hebrew term ébed is used to describe a wide range of socioeconomic roles. (The Greek word doulos in the New Testament is similar, as in Colossians 4:1.)

God wanted them to be treated as family. It was basically a structured mutual aid partnership.

Criticism without research doesn't mean much.

Jesus' dad was pretty adamant He owned everyone.