I can't comment about 4e, my knowledge of that system is very limited. But in 3e, game balance was in part dependent on magical item availability, which was dependent on how much gold the party had. So it was one thing you could change with more difficulty. In 5e, the gm can increase/reduce the amount of reward to their taste with much reduced impact on the power of the PCs (It will have story impact, but less "I can/can't afford this fancy new +2 shield!").Why and how is it suddenly less important than in the real world wrt the shared fiction
Every module sold by WOTC or even it looks like 3rd parties creates actual expectations of character gold and magic items and they really are assuming SOMETHING and they can pretend otherwise but I am not accepting the claim.
However if a game designer admits like how 3e did and 4e did you can then easier adjust to what the assumptions instead of guessing while changing modules. Now this has little impact on those of us who do not purchase modules. I could throw on the inherent bonuses rule in 4e and neglect magic items entirely for a particular campaign AND still use every module coming down the pipe or ones shared by others. And I could give any or no gold (it would annoy my players when they had to forage all there food but hey some would like that I enable foraging for ritual components too)