I assume it's mostly just that women seem to make good targets for this kind of "2 minute hate" in communities that lean male in demographics. It also helps the "super-logical climate denier" vs "irrational hippie climate alarmist" thing that so many cling to even now. Everyone knows young females can't use logic, that's just science fact.

However, I like to think that there's at least some inkling that if a sufficient number of 16 years old are this uncompromising then anyone voting for climate denial today is just storing up problems for themselves in the future as these kids will be the ones paying for their retirement. And if they're paying for a needless climate emergency at the same time, sympathy is going to be in short supply.