While 16 GB of RAM seems sufficient, game requirements will only increase more and more. To future-proof the consoles, going for 32 GB would be ideal, especially if several applications were to run simultaneously. But maybe Sony and the rest don't want to future-proof. Maybe they just want to lower costs and release a new console on a 7 year lifespan.
I'm sure they'll go with 16 for its cost-effectiveness.