I think that 5MWD is a natural behavior for some players in response to DM insisting on hard challenge, harsh consequence and creating a feeling of insecurity.
My personal beliefs are close to this but a couple of degrees off.
In a game system where "common" character failure can lead to significant amounts of player un-fun, like character death, we see a lot more cautious behavior from players than in a system where character failure just leads to other, interesting and fun, forks of the story. In a superhero genre we never head about 15 minute working days, but superheors are
meant to occasionally be captured or lose, and even if they do death is basically off the table except for rather rare and well telegraphed occasions.
So in D&D, a game where one of the frequently expected ways of overcoming challenges, combat, can lead to such a big consequence, the natural inclination is to try to mitigate that risk. It's not a DM specifically being harsh - it's baked into the rules and is a consequence of the DM presented expected level of difficulty.
The funny thing is that while I agree about your statement with it being "natural behavior", it's quite counterproductive. The DM has, as they say,
All the dragons. If the party consistently pushes for a single or few encounters per day, the DM will often increase the challenge of those encounters to keep them interesting. Now instead of smaller chunks that after each is a place to short rest and recover and potentially break off for the day, there is a lot less granularity and that means less control for the players and greater atomic peaks of danger for the characters.