I actually think you're speaking way too objectively. For the record I've been into metal since I was 15 and I went to an all girls school most of my life(besides college), so who would I be trying to impress walking around in band tees or dressing differently in an all-girls school?
Most of my friends (especially my close ones) aren't into metal at all. I've been posting on anime forums and metal forums since… 11 years old (yikes!) and most of them I actually kept my identity hidden except for this one and to a select few people in the ones before this. Being into things most people aren't into isn't a way to get attention and hey I understand the girls who legitimately are interested in this stuff and would like a place where they can express themselves and speak amongst other like minded people. Attractiveness, honestly, is just a facade. You can be attractive but that doesn't mean you have a ton of friends and get a ton of male attention, especially if you're just some weirdo who has little to talk about in regards to most pop culture and views dissimilar to the people around you.