^ did you guys not read my previous post in this thread???
I did.
The OP said most songs, not all. And with Prophets, do you mean the chants chorus? That one has octave playing after the chants, and for each vocal is one chord hit, leaving space.
In Nightmare, do you mean the "life was so simple...?" Same thing there, more space for the vocals. If there are riffs, I usually hear more fill styled things.
This is not to say some of their songs, like the others you mentioned do not have riffs. But the majority are left with more space for vocals. I think the OP was getting at why riffs, like Constant Motions first verse riff style, do not have choruses over it in most DT songs.