I think it's a pretty natural upshot of combining resource-based play with GM authority over pacing.XYZ encounters per day was a bad idea in 3E as well. Idea hung around for to long.
Originally, D&D was not designed around GM authority over pacing. You can see this, for instance, in Gygax's advice on Successful Adventures in his PHB; and in Lewis Pulsipher's essays in White Dwarf around the same time. The GM mapped and keyed the dungeon, but it was the players who made decisions about how to explore it, what risks to take, etc. And there was no particular expectation that all PCs would equally useful in a wide range of contexts. MUs who had been carefully and patiently raised to mid-to-high levels were just more powerful - but still benefitted from having fighters on expeditions, to help protect against wandering monsters in particular.
AD&D retained the same approach to player-side resources even as the common structure of play changed, from player-driven dungeon exploration to GM-led adventures. 3E and 5e are pretty similar in both these respects (and there are innumerable posts in this thread that speak to this model of play). And so it's natural to give the GM advice on how many encounters they should include in their adventure, if the goal is to challenge the players without hosing them.