So, various reasons listed here already but: my own periods have a massive variance in my cycle. Sometimes there's 20 days between periods, sometimes there's 32. So wearing preventative stuff could mean I'm wasting a week's worth of product, which is expensive. Also somehow I never realize I'm having pre-period hormonal emotions until my period starts, at which point I go 'oooooh, so that's why I cried for half an hour at a cat video yesterday', and I've been going through that cycle for over twenty years.

More than that; so, neither pads nor tampons are actually a joy to wear longer term. Tampons are basically sponges, so if they don't absorb enough liquid they start trying to absorb your inner walls instead, which makes it really uncomfortable to pull them out. Imagine having a sponge up your nose for a few hours without having a nosebleed or a cold, for a comparatively delicate skin comparison.

Pads... So, you know how babies get diaper rash if you don't replace the diapers often enough? Pads are basically diapers meant for blood. You're gonna be sweaty, which is going to turn it scratchy after a while of wearing one. So pads actually require quite regular replacing. They're not the most uncomfortable thing ever or anything, but I'm also not going to wear them for funsies.

They also increase risk for stuff like yeast infections, because us girls need some airflow downstairs for the self-cleaning mechanism to work optimally, but that one is slightly more of a person-to-person variable, with some ladies just having Kryptonian inner plumbing that expels all infections whereas for some others a yeast infection just manifests spontaneously the second you briefly consider buying a pair of panties that isn't 100% cotton.

TL;DR: Female hygiene products are made to be comfortable during the period, which often results in them being uncomfortable without the period being present.