You don’t hunt a dragon, or fight a dragon, or slay a dragon. You invade a dragon. Or depose one. Or usurp one. Or destabilize one. But you don’t walk over and kill one.
Dragons just aren’t like other creatures. A dragon is more akin to a force of nature than a giant beast. What could an Alexander do if with a nigh-invulnerable physical form, personal spellcasting, and wings when the fastest man travels on horseback? Oh, and about a thousand more years of life to accumulate, compound, sway, consolidate, learn?
The mistake is in thinking of dragons like any other creature, with a statline and a singular Monster Manual entry. Don’t think of a dragon as a creature, no matter how powerful. Think of a dragon as an institution.
It’s tempting to get this one wrong. We’re people and we think like people. Say you see a tribe, and you want to lead it. Maybe you go over and kick the shit out of the guy in charge, and now you lead the tribe. That makes sense, at the most basic level of human society. The biggest badass gets to be in charge; you kill the chief, you get to be the chief.
This only works because humans are within an order of magnitude of each other. You can be a tough motherfucker, but ten slightly-less-tough motherfuckers still win. A dragon is a different thing. A dragon is many orders of magnitude more powerful than a humanoid.
Even if you were standing next to a dragon, what are you going to do? Hit it with a sharp thing you’re carrying?
The point is, D&D is thinking at the wrong scale. A dragon isn’t just a big beast that you can chase down with enough men and pointy sticks, as though it were a particularly nasty mastodon. To issue forth to bring low a dragon is more akin to sitting in Ionia and plotting how you’re going to march to Susa and kill Darius. Or how you’re going to get to Seoul so you can find your way north and end the Kim Dynasty. In a very real way it doesn’t matter whether you could kill Darius in a swordfight—that isn’t the point.
The point is that you’re trying to bring down an institution, an organization controlling territory and an economy and a society, an institution that may have been successfully fending off interlopers since before your father’s father’s father’s . . . nix that, since before your race figured out the addition of tin to copper makes bronze. And by interlopers, not a plucky band of misfit rogues and swordsmen and magisters, but real interlopers: armies and climate change and pestilence, mana barrens and barbarians and forces bringing other empires low.
Any dragon past an adolescence has satrapies and vassals, an economy and a culture, death squads and intelligence cabals, a court and counsel. And it will have all of these things after the ones it has die off, and their sons and daughters pass from remembrance. Ultimately, institutions can and do fall; but they are rarely slain by cave-creeping good-for-nothings with glowing swords and stage-magician cantrips.