Why do almost no “pro-choice” advocates actually explain why they believe the baby is just a clump of cells?

Surely, that should be their main focus, considering pro-life advocates oppose abortion because they believe it is murder. Yet, instead of addressing this directly, pro-choice supporters often just repeat the phrase, “My body, my choice.” At least pro-life advocates attempt to explain why they believe abortion is murder.
Why do almost no “pro-choice” advocates actually explain why they believe the baby is just a clump of cells?
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