I'd like to ask why. What's the appeal? I mean where's the music? The beat, the lyrical flow? I know it's cliché to say this but isn't it just noise?
I think the easiest way to answer this question is to consider an actual death-metal song. Here are the first two minutes of "Hammer Smashed Face" by Cannibal Corpse.
[yt]http://www.youtube.com/watch?v=fTpQOZcNASw[/yt]
If you listen closely to that clip, you'll discover there are actually five (or maybe six) short phrases or "riffs" being played.
The song starts with the band playing one riff four times in succession (AAAA). Then it introduces a new riff on the bass (B). Then the whole band joins in, playing the second riff three times, with a variation in the middle (BbB). Then there's a third very short riff, repeated eight times (CCCCCCCC), Then they start playing a fourth riff, which is slower and more distinctive, and serves the purpose of a chorus.
What's more, the first four phrases (AAAA) are each arranged slightly differently, so that they build to a climax, before the bass break. (I don't know enough about guitar-playing to tell you exactly what they're doing differently, but the differences are clearly audible) The bass break itself is designed to give the listener a momentary rest, and to build tension, before releasing it with the next barrage of riffs.
Again, if you listen closely, you can also hear the drummer utilizing a similar variety of beats: the opening 1-2-3, 1-2, 1-2-3, then a blast beat, then a slower beat combined with double kicking on the bass drums, then back to the 1-2-3.
A lot of death metal follows this musical collage approach. It seems like there's no melody, because the band is switching rapidly from one melody to another.
Song structure is more important than any particular melody. Any good song is like a good story: it has an initial incident, rising action, a climax, and a brief denouement. And a good death metal song (and I think "Hammer Smashed Face" is pretty good) follows this pattern as well.
Now, you may not like that sort of thing. But I think it's clear that it's not just noise. They've clearly put a lot of thought into what they're doing.