No, that's what the auto-tuner is for

Very few rappers have that strong vocals.

Alicia Keys, Adele, Katie Melua, Whitney Houston,Hayley Westenra...

How many rappers could go up against any of the above? and come out above? as a vocalist if you set them to singing a non rap song?

could you imagine a rapper doing this (live without auto tunner?):

or something like this:

No. They are actual musicians, not pop stars. Being able to sing isn't always as important to metal musicians. And, for the record, PLENTY of them know how to sing. Don't promote what you don't know.

One thing you will definitely find is that metal as a whole has a very large amount of sub-genres that each seek to bring about a unique sound and style. Sometimes it is about pumping your fist and signing along, and sometimes it is about letting the music take you on a journey or tell you a story. Despite the fact that metal has arguably the most intelligent lyricism out of any other genre, you don't always need words to tell a story. In fact, metal is probably the most complex genre there is; so unfortunately I am going to have to call you out on your false title once again.

One genre that places instrumentation above vocals would be post metal. In post metal the music is more about long winded passages of organic instrumentation than it is about vocals - which does not mean the vocalist can afford to be poor by any means - but it does create less need for a good vocalist. Isis' classic album Panopticon is a good example of this. The vocalist isn't necessarily terrible, but he does not sing much, for the music is more about the holistic journey of organic sound the listener will be taken on. Almost like guitars talking to each other.
Here is that album: https://www.youtube.com/watch?v=xA2AJIiw0p8

Sometimes it is true that vocals are intentionally bad, unintelligable or even held back in the mix to create a unique effect. One such example of this is the Jane Doe album by Converge, which is coincidentally my favourite album of all time. The lyrics are very intelligent, but impossible to understand. This is done intentionally to create emotion and a specific array of effects on the listener. In Jane Doe's genre, the music is shaped around the songwriting, blistering instrumentation and raw emotion of the music - not so much the vocals.
Here is that album, which is not for the faint of heart - https://www.youtube.com/watch?v=zLRYREmJqho

In some genres the vocals are clean completely and quite good in fact. From there they can be taken as clean only or have a little death growling mixed in for added effect. The music and sound mix places more emphasis on the vocals; specifically vocal melody and harmony. Sometimes hearing the lyrics is an essential part of telling the story you want your album to tell. One band who I believe is the epitome of this is Opeth. Listen to their song The Lotus Eater - sometimes metal music can be designed for great vocalists.
You can listen to that song here - https://www.youtube.com/watch?v=XhohQNdSt7g

Anyhow, if you have any other questions feel free to ask me

Fuck no! I'd like to see someone scream from their diaphragm without ruining their vocal chords. It's also really not easy to sing clean right after screaming. I can never see myself singing metal in a band at any point in my life, I'm pretty sure I'll just stick to playing drums.

Jesus, Fred Durst is a decent singer? Limp Bizkit is metal? LOL. There is too much wrong with the OP for me to take it seriously. You really need to broaden your "metal" horizons.

Radiohead LOL.

Aren't Radiohead an Alternative Rock band?

Stone Sour's vocalist (also the vocalist of Slipknot) Corey Taylor is an excellent vocalist.

A more soft, melodic song: https://www.youtube.com/watch?v=GIzDsGyxsQM

A heavy song: https://www.youtube.com/watch?v=_tmxCRRmNhM


Avenged Sevenfold also has a good vocalist.

A melodic song: https://www.youtube.com/watch?v=A7ry4cx6HfY

A heavy song: https://www.youtube.com/watch?v=HIRNdveLnJI


There are many Heavy Metal bands with talented vocalists. Even Marilyn Manson deserves some credit here:

Heavy: https://www.youtube.com/watch?v=w5LZ8YErl2M

Melodic: https://www.youtube.com/watch?v=X9BEjwI5DJg

Hell no. listen to the album Around the Fur by Deftones. Some lovely vocals here. First album by 30 seconds to Mars is decently heavy too. Good vocals here also.

You've declared an uninformed opinion on a subject you know nothing about (Radiohead? Really?). You were doomed before you even posted this. There are so many different kinds of metal out there, and from what I can see you somehow managed to scrape the bottom of the metal barrel when the barrel was already full. Bravo.