According to metalsucks, it's because metal singers are big babies. Don't get mad and stop reading, I don't mean that literally. Well, wait ... I kinda do.

According to the article, Dr. Krzysztof Izdebski of the Pacific Voice and Speech Foundation says screaming without hurting our vocal cords is a "lost ability" that we all start out with:

“A little baby has all the sounds — it has the sounds of scream and growl, and inhalation and high pitch and whistle and low pitch, and so I thought, wow, you know, all of this we have, we learn from the beginning. We have it and then we somehow lose it, and the patients who suffer can’t learn how to do this.”

Bottom line, rugrats can scream all day without hurting their vocal cords and metal singers have retained this ability while most of us lose it. How? Because the sound isn't dependent on various "throat parts", (aryepiglottic folds, arytenoids and epiglottis for example), crashing into each other. Simple turbulence creates the sounds with little or no contact between the working parts.

The videos makes that all a little clearer but, now you know...