Why isn't abortion considered murder but a pregnant woman being killed is double homicide

Because it's about consent. If you kill the unborn child of a mother who wanted to be pregnant, then you have violated her consent by terminating her wanted pregnancy against her wishes, which is bad; whereas if a woman gets an abortion, she is utilizing her own autonomy over the decision of who can or cannot use her body, which is good. It's not an inconsistency when you understand consent and bodily autonomy.