For fear of of sounding like an in-my-day curmudgeon, are DMs these days just too lazy to make the game their own?
You're coming off as
Old Economy Steven here.
In the 1970s, most games of D&D were focussed round the dungeon as somewhere to explore, loot, and conquer. Writing a dungeon is almost certainly the easiest form of adventure writing - it's a self-contained environment that has to have ideas in it, but doesn't need to make much sense. And there should be plots but they can be cooked rare as the PCs are going to go through them with all the subtlety of a chainsaw through balsa. There wasn't much in the way of mechanics because a statblock normally took up a single line, and by your second adventure you could fake it easily. And still prefab dungeons were pretty popular. (Hexcrawls are possibly even easier to write than dungeons).
In 2015 dungeons are considered a charming diversion rather than the unit of gameplay. I'm not expecting My First Dungeon to keep the party busy for a dozen sessions.
To make my point about statblocks, I'm comparing the Owlbear from the 1E MM with the Owlbear in the basic rules below. And how many numbers each of them has that relate directly to game mechanics. Yes, leaving off the stats is fine - but a newbie DM doesn't know that. (And even a newbie DM can quickly realise that they need Hit Dice, Move, AC, Attacks, Damage, and size - with the rest being about the environment other than the hug and size)
Owlbear
Large monstrosity, unaligned
Armor Class 13 (natural armor)
Hit Points 59 (7d10 + 21)
Speed 40 ft.
STR 20 (+5)
DEX 12 (+1)
CON 17 (+3)
INT 3 (−4)
WIS 12 (+1)
CHA 7 (−2)
Skills
Perception +3
Senses
darkvision 60 ft., passive Perception 13
Languages —
Challenge
3 (700 XP)
Keen Sight and Smell.
The owlbear has advantage on Wisdom (Perception) checks that rely on sight or smell.
Actions
Multiattack.
The owlbear makes two attacks: one with its beak and one with its claws.
Beak.
Melee Weapon Attack: +7 to hit, reach 5 ft., one creature.
Hit: 10 (1d10 + 5) piercing damage.
Claws.
Melee Weapon Attack: +7 to hit, reach 5 ft., one target.
Hit: 14 (2d8 + 5) slashing damage.
A monstrous cross between giant owl and bear, an owlbear’s reputation for ferocity and aggression makes it one of the most feared predators of the wild.
So first we have the fact that it's far far simpler to make "My first dungeoncrawl adventure" that mirrors the default style of play than it is to make something that resembles Tyranny of Dragons - and a dungeon crawl is less likely to go off the rails. Second, if running by the book it's far easier to make an old school monster. Third, the AD&D owlbear, despite hit points in the 20s, will last longer than the AC13 5e one so there's more play time for the prep time that way. Fourth, oD&D relied a lot on wandering monsters - which provided content for almost no prep.
Overall I'd estimate that it would take a newbie DM starting from the Red Box somewhere around a fifth of the time to come up with a four session adventure than it would a newbie DM starting from either the basic set or the PHB/DMG/MM - the environment's simpler, the monsters are simpler, they take longer to deal with, and you don't need to fret about the players doing weirdness to upend the plot as much. The gap narrows a lot for experienced DMs - but we aren't talking about experienced DMs here.