Where are you getting that? The roles of the DM and the players are explained in the PHB and in the Basic Rules document, and there's an ultimate goal of having fun by following those roles, but I don't see anything about violating those roles in pursuit of that goal.
If you're playing baseball, and one team decides to go easy on the other team because they're losing so badly that it's not fun anymore, then you can hardly call that a proper or expected way to play. It's not the job of the better team to try and make the other team feel better, and the inferior team may even be offended that they would stoop to pandering like that. Likewise, it's not the job of the DM to violate the process of play in order to make things more challenging for the players, and the players may well resent the DM for cheating in order to do so.