Realistically, weapons too large or unwieldy to sheathe were simply carried in-hand until they were used. Realistically, anyone wielding one had a sidearm sheathed on their hip, as well—which is what swords often were, something to carry when you couldn’t bring your weapon-of-war around, or if you lost that in a battle.
Also, just to be clear, people sheathed things at their hip larger than you might expect. We’re talking about polearms and only the very largest swords here. It’s not exactly clear what D&D means by “greatsword”—D&D gets the names of swords wrong constantly—but considering its lack of the reach property, it probably isn’t referring to the kind of sword that’s so large you can’t wear it at the hip (which was a very unusual and specialized weapon not used in most of history—we’re talking about basically a polearm made in a sword-shape, which makes it vastly more expensive and heavy than a polearm ought to be, so that kind of weapon was not very popular). Certainly, the swords that the real world called “greatswords” could be and commonly were sheathed at the hip.
D&D doesn’t really implement any of that, though. Back-up weapons are somewhat unusual, and for the most part consist of a back-up ranged weapon for a melee warrior or a back-up melee weapon for a ranged attacker. Due to the systems’ focus on specialization, and the importance (at higher levels) of magic weapons, switching to a back-up weapon can be close to surrendering. And since the system also doesn’t model the difficulties that very-large weapons have in close spaces, and few tables bother to object to adventurers casually carrying weapons of war around, it’s almost never used for sidearms.
And then we get to back sheathes. Hollywood and gaming has people strapping weapons, particularly big ones, to their backs all the time. You could be forgiven for thinking sheathes across the back were the norm. But historically, back sheathes all-but-didn’t exist. A lot of historians would tell you that they simply didn’t exist, and indeed it’s hard to contradict them with any solid evidence. In fact, even quivers across the back—while they did exist—were vastly less common than quivers at the hip, which I don’t think I have ever seen in a game or movie.
Also, for the record, assuming the same construction, a back sheath is limited to smaller weapons than a hip sheath. This contradicts typical representation, where back sheathes are used for larger weapons, but it’s simple mechanics: in order to clear the sheath, your hand holding the weapon has to be further from the sheath’s opening than the length of the weapon above your hand. For a sword at your hip, that is potentially the length across your torso plus the length of your arm—and yes, people did sheathe things that nearly large. If the sheath ends at your shoulder, then you are limited to the length of your arm—and even then, it’s really hard to unsheathe a weapon like that, and all-but-impossible to sheathe it again. If a weapon was ever strapped across the back, it was being transported, not readied for use.
But D&D isn’t set in our real world’s history. You can construct back sheathes differently from hip sheathes, so that they are functional with weapons of similar sizes (in short, a portion of the sheath cannot be a complete loop, but open on one side, so the blade can be slotted into it before sliding it down). You could probably even come up with something that works, more or less, for a polearm (though your typical polearm is somewhere around eight feet long, and they can be larger than that—at some point, no matter where you put it, it’s not going to work). Magic, obviously, can simply solve all of these problems.
And the society in D&D is vastly different from any found in our history—the very notion of adventurers as found in D&D isn’t found in history (Ireland’s fianna are the closest I’m aware of). Maybe in a world filled with monsters, it isn’t that weird to casually carry around a weapon of war, even in town. And certainly, in a world of magic weapons, it’s entirely plausible for warriors to stick with what would otherwise be a suboptimal weapon since that’s the one with magic on it. Tossing your polearm aside to draw your sidearm when the enemy gets close in is just sensible in reality—throwing away Gungnir, though, might not be so sensible.
Ultimately, the rules don’t really speculate on any of this. These are a lot of setting details, and a lot of it is going to be table-dependent—how simulationist a game do you want? How picky do you want to be with these details? All the rules have is “item interaction.” Which, really, seems to me a fair compromise—maybe it’s interesting for the game to have PCs sometimes not have their weapon in-hand while doing their dungeon-delving, and there ought to be something for it getting there when it’s time for dragon-slaying, rather than it just teleporting. So... item interaction. Done. You could implement more for your particular table if you wish (but at least some of the time that should realistically be “I don’t need to do anything at all, because I’ve been carrying this spear in my hand the whole time.”), but that’s up to your table.
But let me caution you on that: rules have costs. When you implement a new rule, you have to work it out, you have to write it down and edit it for clarify, you have to present it to the table and ensure everyone’s understanding of the rule, and then you have to devote game time to enforcing it. All games, by necessity, skip over some of the details of real life; this is known as abstraction. And the choice of abstraction is a very important design consideration for a game—and never a trivial one. There are always trade-offs. If you dramatically ratchet up the game’s simulation of sheathing and drawing weapons, you are deciding to make that a bigger, more important part of your game, that takes more of your time and attention than it currently does. That ultimately means less time and attention for other aspects of the game. You should think carefully about what the costs are likely to be with any changes you implement here. Consistency in abstraction, too, is important—if drawing and sheathing is so detailed, why aren’t other aspects of reality? Are you going to reduce the abstractions for everything, across the board? You’ll have a better simulation of reality, but that may very well come at the cost of having less of the fantasy that people play D&D for.