Being aro is generally treated this way because of the whole amatonormativity thing. There's also another problem: lack of visibility in the LGBTQ+ community. Just, look. The most visible and loudest part of the LGBTQ+ community keeps yelling about "love is love", not even noticing us. And if they already notice us, they see only aroaces, while aroallos are still being demonized.
I'd say, there are some non villainous characters in the media that could be interpreted as aromantic, but the group is very small (not saying about the actual representation) and possibly rather they're side than main. However, there's one interesting example – Kawaki (Boruto). Kawaki is one of the main characters, he isn't a villain and seems to at least be repulsed by romance (and it's not only my observation). I'd totally "buy" him as aro (but maybe just because of him being one of my faves).