In Olaugah Equiano's Narrative of Life he speaks passionately about slaves being separated from their siblings. Why would the purchasers would not want to keep siblings together if they had the necessary funds.

In Olaugah Equiano's Narrative of Life he speaks passionately about slaves being separated from their siblings. I wonder why the purchasers would not want to keep siblings together if they had the necessary funds. I understand that many times family members on a plantation were separated from each other as punishment, but why would the purchasers initially separate them? I would think that it would increase productivity to have family members together.