To seriously answer the question, it's due to, initially, 2nd wave feminisms push for "female empowerment."  Basically, the idea is that all beauty standards (grooming, hygiene, physical health, etc.) were/are implanted into the collective culture by the patriarchy, and therefore, rejecting those standards is empowering, as feminists are not allowing themselves to be controlled.   But really, its mostly just laziness at this point.