As far as I can tell, the handwave explanation is that seriously posthumanity-oriented civilizations always end up moving away from the material universe in som

As far as I can tell, the handwave explanation is that seriously posthumanity-oriented civilizations always end up moving away from the material universe in some vaguely specified way when they improve themselves sufficiently far. The Culture and other advanced civilizations who are still hanging around are inveterate eccentrics obsessed with sticking close to their biological baseline.