That's just a general design constraint, but it doesn't stop ND from adding volumetric lighting @ 720p, integrated physics system for the entire level on simulated waves (this is beyond linear moving train in U2), seamless switches between gameplay and cinematic views while streaming animation from BD without HDD install, overall great lighting, character model with dynamic animation and improved eye animation, etc., etc., etc.
All this in PS3's limited split pool memory, and so-called slow BR drive.