Because if you were able to predict with better accuracy than I was something that happened to my body on a fairly regular cycle then you’d be surprised as well.
I grew up with a sister that experienced severe dysmenorrhoea. I’m also very familiar with differing length cycles, variable cycles, the unpredictability of them, and the changes that the Wellesley effect can have on the cycle period. But all that said, menstrual cycles are not random, otherwise they wouldn’t be called cycles.
One girlfriend explained that she felt that the periodicity made her feel like a “baby-machine” and like “a mere animal” (her words, not mine) and that not thinking about it was a way of sort of re-humanising herself. I’m not saying that this is what other women feel, but at least in her case, she was actively ignoring her cycle. A practical side-effect of this was always making sure we had bleach, stain removers, laundry detergent and a change of bedsheets in the house.
BTW, I’ve found the grumpiness is the least obvious sign overall: changes in scent (nape of neck and other places), body temperature changes, skin tone and posture are also indicators. All these have low specificity but they’re sometimes better than “Isn’t your period due?” “How dare you assume… <rant> <rant> <rant> <rant> <rant> <rant> oh, my period’s arrived… er can you put a white wash on?”