How are people still with Asuka decades later? There has to be some weird psychosexual thing that makes her so iconic

Probably just has to do with how old the anime is, she is probably a lot of people's first waifu. I also suspect a lot of submissive, meek weebs identify with Shinji a lot (whether they want to or not) and like to self-insert into the scenes the two of them are in.

Last idea: NGE is a "coming of age" anime that features a young boy thrust into close-quarters with an attractive girl his age. It's unlikely most weebs got to experience this irl or did experience it by, again, self-inserting as Shinji when they first watched it, so they personally attached themselves to the "young love" they never got to have.

TL;DR: A weeb's first "girlfriend" was probably Asuka.