Because of course you're going to immediately recognize the fire room coming from a different angle and with bad lighting. I've seen any number of people when lost in large buildings walk well into an area, then look around and go "Wait a minute...I think we've been here before. Oh, yeah, remember that archway?" If that's not you, I'd have to say you're at least a modest exception.
1) This assumes multiple entrances into the same room. If there isn't multiple ways to get to the same room, then you are going to be able to easily identify the room.
2) I think the heat and sound of roaring fire might still be a clue that they've found the fire room again.
3) We are completely ignoring the utility of chalk.
In the old day, you made damn good and sure you didn't stay too long which, oddly enough, required making sure you had a clear idea of your path of retreat.
And in the old days you still died if the dungeon was bigger than you thought.
But again, how often are you going into dungeons of radioactive decay? I've maybe had... one time in the entire run of 5e where we were in a place where the environment itself was actively harming us, and what you quickly realize as a DM is that these areas have to be either small or the damage incredibly minor, or the entire thing is impassible anyways. Even 1 damage a round turns into 10 damage a minute, 600 damage an hour, which kills ANYTHING you put in it. 1 damage a minute is still 60 damage an hour, and you need to be getting into levels 10+ before you can survive that hour alone. And that's ignoring combat.
And if this is the main argument for why you need to map.... that's too niche to matter.
And I was talking about where the habit got started and why.
And I've never disgreed why the habit started. I've disagreed that it is still a major concern now, in 5e.
That'd be true if you had the same resources available at the end as at the start.
You don't need the exact same resources at the start. Sure, you've used a few abilities, but you still have your at-will options, and THERE SHOULD BE FEWER THREATS. Maybe the wizard no longer has fireball and the paladin can't smite, but the wizard still has firebolt and the paladin still has a sword and shield, and with the number of threats decreasing there is less need for their more powerful options.
And mine is you can need to retreat well before you've gotten it thinned out enough for that to be relevant. And again I'm not talking about 5e. I'm neither qualified nor particularly interested in doing so. Note the tag at the top of this thread. It wasn't true in OD&D, and it wasn't even true by the time of 3e.
I've noted the tag at the top of the thread. If you aren't interested in discussing 5e, maybe instead of insisting on telling me why you are correct, you could acknowledge that 5e is different. Because this entire line of discussion STARTED by discussing 5e's mapping rules. Not 3.X's rules. Not OD&D's rules.
5e characters CAN thin out a significant number of enemies. Just last night in one of my play-by-post games we had a level 6 supported by three level 2-3's (single player game with some NPCs) who took out 16 enemies. They are in a bad way after that fight, but they cleared the entire building of slavers and there is zero reason to suspect more enemies will be coming until they can take a short rest and restore themselves. Because the slavers certainly weren't dealing with attacks every hour on their base.