The DM clearly neither expected the player choices, nor the outcome of the dice. (From a metaplay perspective, I'm quite certain that this was not meant to be a combat encounter at all.) The outcome required a lucky roll by the player, and an unlucky roll by the NPC, and was no doubt improbable - but the dice are fickle.
And here in lies the differences between your example and BG3. In BG3, shoving enemies off the cliff is both easy and an expected tactic in many combats. And while it's totally fine to create a game based around shoving enemies off of cliffs (in a sense, smash bros qualifies), the rules and goal of D&D combats (dealing damage over multiple turns to reduce enemy HP to zero) doesn't mesh well with the overpowered and common nature of shoving.
Even Power Word Kill, a 9th(!) level spell with the specific aim of being a OHKO, requires that targets have less than 100 HP. It has synergy with D&D gameplay as you have to whittle down the enemy first. Shove, on the other hand, can occur on the first turn and easily instantly end a combat. All at the low low cost of a bonus action from a single character, which is essentially a free action for classes that don't have other uses for their bonus action.
p.s. Last I heard, Barbarian's Throw Enemy was still a 100% chance to throw (not to hit, but the throw still automatically succeeds). Idk if you can Throw Enemies into pits because there might be a distance restriction, but...
