If Metal is supposed to be a separate genre from Rock, then why doesn't it have its own category?

Metal and punk are sub-genres of rock. They aren't separate genres.

Hip-hop is a separate genre. Jazz is a separate genre. Classical is separate, etc. Meaning, those styles of music aren't an offshoot of rock music in any way. Obviously, jazz and classical predate rock music, and hip-hop originally sprang from Calypso music, and only came to incorporate funk & R&B sounds later in its history.

Metal & punk, on the other hand, are both direct offshoots of rock, and are essentially different styles of it. The first metal & punk bands were rock bands. And, if pretty much any rock band plays their songs faster with more distortion on their guitars, they'll sound punk. It takes a little more to sound metal. But punk is easy for any rock band to do, especially now that there's pop punk (which really isn't punk at all. It IS rock).

Well they can't make a section for every genre.... because then there'd be a death metal section and a black metal section and a crossover thrash...

I think it gets the job done to have it as it is. Some people find it to be offensive to rock for it to be associated with pop... in the end though I think it's best for it to have it like it is because it saves 'space' and you don't have to navigate through each genre if you're wanting to answer questions.. Questions regarding Metal can and still do get answered in R&P.

It'd be kind of cool if the site was more diverse with the sections for music but there are sites specifically for music discussion.

Metal and punk are subgenres of rock, and besides rock is rock no matter the genre

It's just an offspring of Rock... Jazz ISN'T an offspring of the Blues, at least not a direct offspring.

because it's a SUB-genre, as you said.

Why not a separate category for punk, goth, new wave, folk, emo, screamo, crap-o, etc

I don't know, I guess there just to lazy to make another category.

You know metal has been around longer than Y!A right?

cuz skittles.