that's because the rhythm guitars cover enough of the low end.
i'm a bassist myself, but sometimes i can't help but wonder just how important a bassist really is in a death metal band. (note: death metal is my favourite genre)
cause the thing is, in most, if not all death metal songs, no matter how hard i listen or no matter how loud i crank the volume, i just can't hear the bass.
on a good day, sometimes i'll just 'feel' the bass somewhere in the background of the song..