I thought you might find this interesting Trollheart.
Consonance and dissonance - Wikipedia, the free encyclopedia
The link is about consonance and dissonance in music. Consonance is something generally thought of as pleasant, and dissonance is usually thought of as something harsher, or not immediately pleasing to the average person's ears.
I've found a lot of extreme Metal focuses on dissonance at many points during songs. So I thought you may be thinking dissonant melodies aren't really melodies and only really recognize the consonant/easily appreciable melodies as such. Though a lot of extreme Metal can focus just as much on consonance as dissonance.
At the top right of the wiki page there's two sound samples, one a consonant melody and another one a dissonant one. I'd like it if you could listen to both to get an idea of what I'm talking about and would encourage you to read further into the article if you want.
Also, here's an extreme Metal song which I'd like you to listen to, to see if you do find melody in it. The reason I'm posting this is because I've found a lot of people who are new to genres and have only heard select groups can have warped ideas on what the whole genre contains. So I wanted to share this song with you to see if it could possibly change your opinion on extreme Metal.
This song would pretty much fit into the genre of Post-Black Metal.
Janvs - Torri di Vetro - YouTube