People don't bother explaining it because it doesn't matter how it works to them. There are too many things in the game which are difficult or impossible to describe the mechanics for, or which already do not mirror how actual historical devices worked. It's easy to poke holes in the realism of D&D, especially combat, regardless of whether or not you include magic. Furthermore, mechanical aptitude and engineering prowess should not be a prerequisite for playing a character in a game.
Historically, a Heavy Crossbow in D&D has been one that required a winch, windlass, lever, or other drawing mechanism. In every edition I've got that's bothered to describe them (2e and 3e) it has been so, while others merely intimate it (fires every other round in 1e, and once every three rounds in OD&D, Basic D&D allows faster firing, but carries other requirements like 18 Str to do so). 5e doesn't describe weapons, but I think we can assume that to still be the case. The game says that in 12 seconds a high level Fighter with Crossbow Expert can launch 16 bolts from a Heavy Crossbow, and each individual shot will be as accurate as if the same character had taken a minute to make one carefully aimed shot. Without any kind of magic. Clearly, realism is not the focus of D&D combat.
D&D has always walked a line between the game and the narrative.
There are people who view it mostly as a game that just happens to have an ongoing story. And there are people who view D&D as a storytelling exercise that lets them explore their world and its events, with the game just resolving action. Both these views are 100% right. And you can pull elements that support either style from throughout the game and present them as evidence that D&D is either gamist or sim. But, really, the game works to find a medium between them. If D&D leans too much to emulating reality then the game becomes clunky and harder to play, while if it leans too much to being a game it becomes too abstract and harder to narrate.
Crossbows (along with early firearms) are ones of those things that occupy an uncomfortable place between game elegance and narrative immersion. Once the round shifted from 1 minute to 6 seconds loading crossbows became clunky and problematic. They have to be a once/per round thing, but that becomes hard to balance against multiple attacks. If they're higher damage to balance with being slower then they're open to abuse with alpha strikes or people carrying multiple loaded crossbows. If they do normal damage but take a full action to reload then they're just option traps to catch those lacking system mastery.
But people want crossbows. They're iconic.So not including them isn't an option, not any more than making them broken. So crossbows have to bend reality and be a little faster to fire. That's the necessary compromise.
Limiting crossbows to a single shot unless you're willing to spend a feat also seems like a fair compromise. If you lack the feat, the speed of reloading the crossbow becomes irrelevant as you're going to swap to weapons or you're a class that relies on a single attack. And feats in 5e are a big investment, suggesting you're a master of the crossbow, so it's easier to justify being fast at loading, allowing some more latitude in reality.
Plus, crossbows have become a favourite in fantasy action films, being seen as the equivalent of a gun allowing the adaptation of the modern gunslinger archetype. There are not
one, but
two TV Tropes on the subject.
Most of D&D's"unrealisms" work best when you don't pay attention to them. When you don't draw undue attention to loading crossbows. Like hit points or horses somehow being 10 feet wide or halflings having ant-like carrying capacity.
Things still become a problem at high levels, but not every game makes it to high level play. And greater suspension of disbelief has typically been a requirement of high level play. (After all, typically mid-level fighters can shatter long long jump or weight lifting records. Although bounded accuracy reduces that this edition.) The problem of crossbow speeds at high levels is manageable because the character has earned their improbability, or if the DM has a problem they can lampshade it with a "masterwork" or "magical" crossbow. It's a lot less problematic than a first level human that's barely more than a grunt soldier showing up with the crazy ability.
(Plus, it's unlikely the fighter will decide to be the hand crossbow guy. There are just better weapons. That's really more of a rogue (or maybe ranger) schtick. So that's a moot point.)
If there's any ridiculousness here, I'd put the lion's share of it with Crossbow Expert's circumvention of the Loading property and the Fighter's Action Surge, not the fact that it's difficult to draw a hand crossbow with something in your hand.
Next time you're buying groceries grab a basket and go right to the produce section. Throw four or five bags of potatoes in the basket and lift it with 2-3 fingers. Preferably while holding something like your cellphone in the hand you're lifting with. Now imagine doing that with a 5mm thick string.
It's pretty ridiculous. Action surge and the feat just make it more so.
I get the desire to emulate the two gun wielding action hero. Even if that is also ridiculous, greatly reducing your accuracy. Of course, you never see action heroes with double pistols reloading. They just discard and draw two new guns. And reloading a gun is a lot easier than loading a crossbow.
Gun Kata/ Gun Fu is cool and I liked
Equilibrium. And having a D&D game with that tone sounds hella fun. But that doesn't sound like a good baseline. And arguing that the core rules of the game allow should allow is awkward. That's houserule territory.
And, hey,
repeating crossbows date back to 4th Century BCE (go China). Throw in some gnomes and it's easy to justify a handheld version. That's not an issue.
In the end, we allow it because it's not broken (being identical to using one hand crossbow with the feat), not outside the realm of how ridiculous combat is, and, most of all, because one of the players asked for it because he thought it was fun and cool.
Besides, it's not like repeating hand crossbows weren't a fixture of the game at one point....
A hand crossbow is a good rogue weapon, and does damage comparable to the shortbow, but two of them can be used rather than one. But being able to attack twice doubles the chances of hitting and dealing sneak attack damage, which is a great option for a rogue. It's a huge boost to DPR. And unlike two-weapon fighting, the rogue is safely out of melee range.
Allowing two hand crossbows to be used at the same time every round is like allowing a rogue to just give themselves perma-advantage (but that also stacks with regular advantage).
So, yeah, it can be broken.
And it gets worse if you allow the Sage Advice rules wrinkle that, technically, you can attack with the same hand crossbow twice in the same round. Since there's no comparative Rapid Shot feat for regular bows, this makes hand crossbows faster than a shortbow. Which is ridiculous.
(I'll be ignoring a couple things from that Sage Advice article for my game as I'm also unhappy with the Lucky ruling.)
