This is a function of the stock being in line with the barrel, which assists in recoil control. Compare the M16 to the M1 Garand. Without the pistol grip, the M16 would be awkward to hold. Of course, the civilian AR-15 has less of a need for recoil control, as it’s not going to be fired in full auto.
On a more fundamental level, the design and ergonomics of assault rifles are oriented on creating a versatile primary small arm for the infantryman. You can use them for anything from long-range rifle fire to assaulting a trench.
Note that I’m speaking of assault rifles, not “assault weapons,” which is a meaningless term by itself. State laws have differing definitions of “assault weapon.”
Well, the M-14 (the mini-14’s daddy, chambered in 7.62 mm NATO… which is also the equivalent of the .308 cal, a well-loved deer-hunting round) is still in service. Often, it’s got a synthetic stock. It’s not usually capable of fully automatic fire. So, bottom line is that yes, Virginia, there are military weapons that look like that.
In terms of “styling,” yeah - style sells in the civilian world. Typically, the military couldn’t give two sh*ts about what a weapon looks like - dependability (accuracy, failure rates, maintenance, multi-functionality, etc) are important (cost, too). The civilian market is driven in part by the “I want a cool looking gun like I’ve seen in movies” factor, but also by the fact that the huge amount of money invested by the defense industry means that those “tacticool” guns are, in fact, good at what they do. Pistol grips, threaded barrels, flash suppressors, muzzle brakes, extended mags, short barrels, synthetic stocks, and so on all have legitimate value (even for hunters… although I’m pretty sure no states will allow you to hunt elk with a bayonet ).
Heck, we still mount variants of the 150 year old Gatling gun on all kinds of platforms. Is that an assault weapon (I would love to see a chain gun with bayonets on every barrell, just for the giggles). Not much point in adding a flash suppressor… pistol grip would be worthless… sub-16" barrel would be a joke… But despite lacking those features, I’d rather have a chain gun than a an AK variant or an M-4 in a lot of situations.
At the end of the day, defining “assault weapons” based on aesthetics is about as useful as saying that cauliflower is smarter than broccoli (heck, Mark Twain said so… but he was smart of enough to do so sarcastically).
One of the things to bear in mind about “assault weapon styling” at least in some specific cases, such as AR15/FAL/AK variants, is that if the weapon in question was originally designed to post-WW2 milspec (thus flash suppressors, pistol grips, modular upper receivers, accessory mounting rails, etc.), why would the maker have to strip all but the action and barrel to change it into the “traditional” (“musket”?) form-factor for the sake of the civilian market? The “demilitarizing” requirement is to remove the select-fire capability.
OTOH, though, I do think it is a bit silly when you take a perfectly fine weapon that comes from the factory in the “traditional” form-factor (e.g. a Mini-14) and go all Pimp-My-Gun on it, when they’d hardly ever have to use them in tactical environment and often these mere cosmetics would not hold up too well in real battle. Just as I snicker at guys driving with rims making up half the value of the car, woofers more powerful than the stock engine and suspension lowered to where a speedbump will stop them cold. But hey, it’s their money. (And BTW, real armies don’t put 100-round drums on their grunts’ assault rifles. Those are for machine guns, the rifle’s got fast-change 30-round mags.)
In the 80s a number of makers were doing “tacticool” looking weapons for just the looks w/o real performance, like the TEC 9, a knockoff of a submachine gun design but functionally a cheap POS pistol. Ironically the 94-04 “Assault Weapons” ban helped weed out many of these, leaving a market with actually better, if more expensive, hardware.
(Hmm… now I wonder: In how many of the recent-era sprees, drivebys, etc. are the weapons of choice “tacticool” vs. “traditional” form factor? Does your average violent rampager know that he can do just as much damage, and probably more reliably, with a stock Mini-14 vs. with a knockoff AR lookalike? Or would he feel inadequate?)