I don’t think a low libido is particularly abnormal - or at least one that fluctuates. It’s more unusual I would have thought to be constantly up for it all the time, once you get past a certain age anyway.
As PP has said, the idea of ‘duty’ or ‘maintenance‘ sex isn’t about never wanting sex, it’s just that you don’t always want it at that particular moment just because your partner does.
But if you’re saying why is it seemingly common for women to want sex less frequently on average than their male partners, the reasons you listed would all certainly do it. Other reasons could be -
Normal hormonal fluctuations/ menstrual cycle/ menopause etc. Personally I couldn’t be less interested during my period, I know some women still enjoy sex during that time though
Pregnancy, childbirth - birth injuries or trauma, loss of sense of self/ confidence after childbirth, breastfeeding, feeling touched out from child(ren) all day or that they can’t take another person wanting something/ making demands of them, being scared of getting pregnant.
Being exhausted from taking on the majority of domestic labour and/ or childcare - feeling resentment or losing attraction to partner as a result.
Stress either at work or elsewhere, anxiety, depression - medication for any of the above affecting sex drive.
Am sure there’s loads more possible reasons too.