The same reason you can’t directly control your heartbeat, blood pressure, or how much you’re sweating.
Some processes are regulated by our autonomic nervous system. This system is not consciously controlled as a default because it’s more efficient to be regulated by hormones and other neurotransmitters. Ultimately, like most things, it’s “good enough” to get us to reproductive age and have babies, so it never got selected out of this pathway.
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