I think it's because menstrual product gendering (like all product gendering) is a form of marketing. And marketing tries to appeal to "target markets". And since 98-99% of those who consume menstrual products identify as women, marketing agencies go "oh awesome we can lean into the girlboss angle, celebrating femininity angle, that will get all the women to buy from us".

That's why toys are pointlessly gendered (because gendered marketing sells more products overall than if all toys were considered unisex). Why yogurt is pointlessly gendered. Basically why any product is gendered. It's for marketing reasons.

As time goes on and the market of trans, intersex and otherwise non-cis customers makes itself apparent to these menstrual product companies, they will start changing their language to sell more. Unfortunately profits is the only thing that will change this. :(