Why were slaves considered property?

Slaves as far back as history records have always been treated as property due to the possessive nature of slavery.

A slave, by definition if forced to work for someone against their will. By that very nature, the slave is owned by the person or people that enforce their will upon the slave. In a dark twist, in history, this hasn't been ideal for the slave owner as a matter of liability for their slaves. (e.g. A slave escapes, steals a horse, horse is killed in the process of capturing the slave, and the owner of the horse sued the slave owner for damages.) Twisting even further as this also placed a limited check on the slave owners to make sure that slave riots\rebellions did not happen as they may be held liable for damages. For some slaves this may have improved conditions (to deter running) and for others worsened conditions (more brutal tactics to prevent rebellion).

As a matter of law, when two slave owners would be in diagreement over a slave, the possessive nature of slavery more then likely made treating a slave as a piece of property a matter of legal convience and reuse existing livestock law.