Because several things move at once. Falling estrogen brings vaginal dryness and discomfort that can make sex painful rather than appealing; hot flashes and night sweats wreck the sleep that desire depends on; mood and energy dip; and life stage and relationship factors sit on top of all of it.
That layering is the whole reason low libido is frustrating to treat with a single lever - it is rarely "one hormone went down" and more often "four things went sideways together." Naming which factors are loudest for you is the first genuinely useful step, and it is a conversation a clinician is equipped to have.

