Up until the mid stages of the second world war, armies did indeed look like huge masses of coats and rifles, but that was almost 70 years ago, and what worked back then is now horribly outdated.
Small, self-contained, mobile, properly outfitted and trained units are called modern warfare. In warhams, we say MSU - multiple small units.
A 'real' unit (american army) is five to twelve people, mounted aboard an APC (rhino), an infantry fighting vehicle (razorback, chimera, wave serpent), or an armored car (trukk, landspeeder storm).
Every single person in the unit wears armor, wields a carbine or battle rifle with underslung grenade- or shotgun-module, and comes blinged with essential equipment, tools, ammunition, etc.
Those who don't are specialists, packing support weapons, extra gear - so on.
It's true that once upon a time, infantry with coats charged machineguns, but times changed - a universal realization, if you will.
The world recognized the stupidity in rushing waves of unarmored, almost untrained, inexperienced, ill-equipped men against machinegun nests, because it cost thousands of lives to gain a meter of ground.
Yeah, the russians had their huge armies of conscripts and untrained tank crews, and didn't see a need to change this even when WWII broke out.
Yeah, they lost TEN MILLION!!! men in their war against germany.
Meanwhile, germany 'only' lost five million men. This while being gangbanged by pretty much everybody else in the world, except japan.
Just how did a tiny, tiny country manage a 'mere' 5 million losses against two superpowers and all their friends? Very simple, actually. The tiny country invented modern warfare.
Rather than employ a gazillion inexperienced, untrained conscripts, they had actual soldiers. Tanks were state-of-the-art, and at the start of the war, invincible to allied attacks. The plan was to attack very fast and unexpectedly, with a punishing initial aerial bombardment - blitzkrieg.
This style of warfare is mirrored in the vanilla marine codex. It's what they do both in background and on tabletop.
No, you don't need to know this, but it's fun to point out whenever MSU is brought up, and someone starts talking about 'real armies.'
I could go on about it, but enough's been said already. Massed infantry formations are a bad move for real, and a bad move in warhams.
"But all those little units are such easy killpoints." So no more whining about it being cheese? Uhh, okay.
6x5, versus 1x30 , equals the 30 being able to kill one of the 5-man teams a turn.
Yes, even though the mass can potentially kill all six 5-man units with perfect rolling, it's still limited to shooting at one target a turn. In short, overkill comes to bite you in the ass once more.
The small units, meanwhile, can move around as they please, and either shoot your huge mass, or run onto objectives.
Sure, you'll eventually wear them down, but since it takes you a minimum of six turns to kill all of them, you're not even guaranteed to do it in a standard game.
If we're dealing with troops versus troops (likely; few other units come in large sizes), the dinky squads are an obvious problem for the mass, because, as hinted at above, it can only kill one of them a turn.
'Hidden' problems also exist - such as the tiny squads all getting a lesser boss who can take killy gear, and, most of the time, the little squads themselves are allowed dangerous guns or implements.
Six units tend to carry six special weapons in total, while the huge blob won't get nearly as many.
The six small squads can target six separate enemy units (meltaguns versus six transports, anyone?), while the blob has to shoot all its firepower against the same target. Overkill, again.
Multiple units open up the possibility of having more transports. Should all six squads take a transport each, the mass now has to destroy twelve units - not six.
Yes, it's true that five models don't have that much of a life-expectancy, especially if shot at - or charged- by thirty, but thirty versus ten would end the same way, too.
So, let's sum up.
You get more total units. More total units equal more special/heavy weapons, and squad leaders. Each tiny unit often lets you buy a transport of some kind. Small squads are no more survivable versus huge units than slightly larger squads.
Not only is multiple small units faster, more mobile, and more killy - they're also more flexible.
Your huge squad can only occupy one part of the battlefield, but the smaller ones can spread out. Why would you take the huge unit again?
Oh, right - 'fluff.'
Does fluff justify it? For some armies (imperial guard, tyranids, orks, demons), yes.
Space marines (all space marines), tau, the inquisition, eldar, dark eldar...do not have fluff that talks about massive hordes. In total, 11 of the game's armies are 'min-maxers,' even in the official background.
Wait.
Does this mean everybody who's accused mechanized space marines of 'not being fluffy' are wrong, and little more than sore losers? Yes, that's exactly what it means.
Same deal for those who've whined over blood angels, templars, dark angels, sisters of battle, chaos marines, gray knights, dark eldar, eldar, tau, or space wolves 'min-maxing,' coming in compact elite forces, or running the dreaded MSU-style.
As for it being 'cookie-cutter' - well, would you rather play horde versus horde until the day hell freezes over? "YES! OH, GOD, YES!" There's another very important thing that people forget about MSU, one that leads to much whining when the aforementioned foot hordes clash: time.
Compared to ten~ vehicles/small squads, it takes a significantly longer amount of time to shuffle large, unwieldy infantry hordes around, and games become tedius, droning, and boring descents into mediocrity very quickly. Yes, you have your giant mass of slow, green boyz, now it's my turn.
Oh, look. Your boyz are no more. How totally new.
Predictable slogs into midfield, where the horde meets a sudden end, aren't exactly stimulating, or even cheap to stage. After all, those dudes cost more than the handful of vehicles, mounted infantry, or small, rock hard squads you should be fielding.
Rule of thumb: no matter what game, sport, or theater of war you're in, multiple small units crush enormous hordes.
In games where armor's got obvious mobility- and combat-advantage over infantry (like in reality), and even the lightest vehicle (almost everybody's got transport-access) is considered a 'hard' target, only those who really hate change will stick to foot and hordes. In the end, they've themselves to blame for not playing to the game's strengths, or understanding the system.
Footdar. Really? That's the most unfluffy combo in the game.
And you call yourself 'a social gamer.' Get out of here, you WAAC jerk.
By the emperor, at least three quarters of all armies posted on forums are large foot hordes, with no flavor units, and lots of troops.
Talk about cookie-cutter.