I’ve seen many people here argue it’s hard.So I haven't seen anyone here characterize DMing D&D as hard. We've said it's harder than being a player and this is a fact. Saying that we are arguing that DMing is hard and then constructing a counter is a classic Strawman.
Take your 3 room dungeon for example. The DM had to create a dungeon. The players didn't. The DM had to come up with the encounters to put into the dungeon. The players didn't. The DM has to describe the evironment. The players don't. The DM has to run 6 goblins, deciding where they move, who they attack and how they attack, if they run, etc. The players have to worry about 1 PC each. The DM has to adjudicate all actions the PCs take. The players adjudicate nothing.
DMing is not hard. It is in fact harder than playing.
First you have to decide if the outcome is in doubt. You don't even get to a roll until after the DM completes this step and decides that the outcome is in doubt. Then the DM needs to figure out if there are any bonuses, penalties, advantage or disadvantage that apply to the roll, and often those things don't come from a player specific ability. THEN the DM comes up with the DC. If the action isn't specifically covered by the rules, which happens fairly often in 5e, the DM has to come up with even more in order to adjudicate things. The DM also needs to figure out if the action in question is going to use the typical stat for the skill, or use an unusual stat. Afterwards, the DM narrates the result. The player just declares what he wants to try and do, and hopefully how.
Not according to the 5e DMG, page 5. That page gives a very clear example of the DM's job as master of rules including correcting a player error on movement.