Blue
Ravenous Bugblatter Beast of Traal
They don't understand that encounter difficulty (hard, medium, deadly etc) in 5E is predicated on the expectation that players have to deal with around 6-8 of them (defeating each one) before recovering long rest resources (hit points, hit die, spell slots, arcana, rages, sorcery points).
Bad DMs allow the 5 minute adventuring day either through ignorance or laziness. This throws encounter balance totally out of kilter as the players are able to nova (dumping all long rest resources) the single encounter, trivializing it. In response to players Nova-ing an encounter, the bad DM then proceeds to ramp up encounter difficulty to Deadly+... thus forcing the players to optimize and employ nova tactics, or else die.
I totally agree with your point that 5e absolutely relies on attrition vs. recovery mechanic and if you ignore that the game balance will be off, leading to too-easy encounters.
Even "deadly+" encounters can become easy when it's your only fight for the day.
However, I disagree strongly that it's a "bad DM" who allows it. It's a near-fatal flaw in design (and I love 5e, I'm not picking on it) the does not allow the DM to match the resource recovery to the narrative needs instead of skewing the encounter frequency to a forced stop to meet mechanical needs. A DM should be able to throw one encounter and have it work if that's what the narrative demands.
I hear advice like "put in a time limit" so PCs can't rest, but I hear that as "every single adventure, every single day with any encounters, there must be pressure supplied for 6-8 encounters". And that's not a reasonable stance. Sometimes, sure. Every single day that there are any encounters over the PC's entire lifespan? Pshaw.
Of course, this also goes back to surviving == winning, so you need attrition to potentially kill PCs. Adventures and encounters with other win conditions - stop the ritual, save the commoners that are being eaten by ghouls, protect the king - these aren't reliant on PCs being worn down for them to lose. But again, that should be "when the DM wants/when the story dictates, not as a workaround for straightjacketed encounter balance because of resource management.