But you don’t. Take children, for example.
A child comes from the bodies of two people who ‘donated’ their genetic material, obviously; but those people don’t ‘own’ that child as property - legally, we are not allowed to sell our offspring, or intentionally harm them just because they “belong” to us.
(Just as it’s not legal to kill oneself, although that may not be the best example to use.)
Agency ≠ ownership; possessions are owned, whereas people are living beings capable of making their own decisions about their lives.
It may seem like mere semantics, but words are symbols for ideas; they matter.