You are a tribe that has recently begun a labor-intensive agricultural form of existance. A tribe the next valley over sees your farms and raids your village. They kill your warriors, rape your women, and streal your food.

The next time they come back, you are ready, and you kill a bunch of them. Most flee, but a few are too wounded to run away.

What do you do with these people?

1. Let them hobble away/escourt them back to their tribe, where they will presumably regain their health and then raid your village again.
2. Kill them and be done with it.
3. Restrain them so they can't raid you anymore but also so you don't have to kill them, as they are begging for their lives.

If you choose 3, then what? You have some kind of cage or shackles for them, and they do nothing all day while they are brought food and water? No one in the village will stand for that while they are going out and hunting, foraging, and farming. So what do you do with your prisoner? You make them work - probably doing the shittiest jobs - in order to earn their keep instead of getting their head smashed with a rock.

Hence, slavery. Then, do it for several thousand years, and it is part of your culture.