Flamestrike
Legend
The 5MWD exists because the system rewards it. Players can try to take full advantage of it or exercise restraint for a variety of reasons, and DMs can try to block or compensate for it. If it were a wholly encounter-based system, for instance, there'd be no impetus towards the 5MWD (it could still happen, but it wouldn't make a difference).
The reason the system rewards it is because D and D is, at its heart, a resource management game. Mechanics im talking about here.
But yeah, otherwise I agree. They could have balanced encounters in isolation of the adventuring day, with the expectation that every group of characters hits every encounter at full strength. The current rules reflects the majority of play styles and campaigns, where on your average adventuring day most groups of characters will do several encounters in a row (ie your typical dungeon).
I hazard a guess that most campaigns feature single encounter adventuring days of Overland travel, and then 'zoomed in' action at dungeons and similar areas, where most groups would face half a dozen or more encounters in a single adventuring day.
Or to put it another way, I can't think of too many adventures or modules that feature a single encounter a day, followed by a rest, another encounter the following day, and so forth as it baseline. This might happen occasionally, but most of the action happens in zoomed in dungeons, ruins and other closely packed encounter areas.
The current maths of the game (and assumptions with encounter design and resource management) reflects this expectation.