Simple. Your typical dragon lives for a bunch of centuries at least and if you're made of - real or alleged - powerful and prized ingredients and are said to be a happy owner of (again, real or alleged) sizeable mound of valuable shinies, then you need to be pretty fucking smart to not only ever hope to make it anywhere near your species' typical lifespan but merely to not go extinct by the time pesky two-legged mayflies figure out how to smelt iron. And given how good humans (and presumably other humanoids) are at being crafty assholes if any combination of survival, money, status and girls is at stake, by the time you reach your adulthood you are going to be extraordinarily good at detecting, thwarting and most of the time preempting constant attempts at taking your life, stuff and good part of your anatomy, some by force and most by guile. Given natural magical aptitude that includes becoming really good at it as well.
So, from adventurer's pov a dragon can be expected to:
- most be likely smarter than anyone in the group
- have innate magic talent
- have at least an order of magnitude more of accumulated continuous experience, including experience with magic
- have gone over and proofed itself against countless variants of whatever you may come up with against it, with multiple layers of contingencies and stuff
- have actually defended against sizeable portion of those in practice too - successfully
- have keen understanding of humanoid psychology, at least regarding how they go about trying to kill stuff
- harbour at least deep distrust of humanoids
- still possess enough raw physical power to casually flatten, roast or chomp in half any party member dumb enough to try to fight it face-to-face
- be able to fall back to high altitude bombardment if things go sour.
Most of those apply to other high powered beings characterized by high intelligence, long lifespan, and often antagonistic relations with humanoids like demons and so on.
All in the monster manual champ, dragons arent that smart either, lizard brain holds them back i guess. Still, the smartest of them are genius, but nothing where the DM is forced to meta to make up for it, and nothing beyond human capabilities unless we are talking very extreme cases from the best races, and even then, not that high. They arent that strong either, for their size (and they were even weaker on earlier editions). An ogre is usually stronger, while being much smaller. All dragons are very clumsy, about as graceful as a peasant.
Their understanding of human psychology is about as indepth as their understanding of the rest of the world, remarkably wise creatures as a rule, but nothing beyond human capabilities either, if they even bother to study them, most of the dont, for the same reason most humans dont study sheeps or ants.
Your typical dragon is lazy, proud and overindulgent, on top of having a remarkably high life expectancy, what they achieve in 100 years a disciplined human can achieve in kist a few. They just start further ahead and have a natural talent for it, which further feeds into their overbloated egos. Add to this the boundless greed that afflicts most of them and they spend too much time trying to get valuables and setting up defenses that they dont have time for much else but their ever important long naps.
Also a dragons wings are the first thing you should take care of, they are brittle, fragile and too much of a tactical advantage to be left alone. CRPGs have to make concessions, the player cant cripple their wings because of lack of game mechanics, but the dragons wont usually fly away for the same reason.
Running the fuck away doesn't quite count as winning. If I can force you to run for your life I can coerce you to do shit I want or at least stop you from doing shit I don't want.
Why be so extreme? you can merely reposition. Its not either stand your ground or flee bro. battles have a bit more depth than that. You can often chose to fight the battle on your own terms, youll live longer if you do.
Because they don't have DM so you can't offload shit onto him.
Which is why you set the world state and let the players do as they will. If you are so inclined devote resources to an evolving world, but to be honest adventuring parties are usually a lot more active than anyone else, and they risk a lot more, therefore they are rewarded with a lot more, knowledge, wealth, glory and a lot of times premature release from their mortal shells. NPCs are largely risk free and have already obligations and set positions, therefore they shouldnt be improving at a noticeable pace, if at all. There are exceptions of course, usually because the story necessitates them.