These days, not many people can say they've seen a real dragon. Indeed some people do say that, and it's not my place to judge if they are lying, having hallucinations, they've seen something else and thought they've seen a dragon or if they really met a real living dragon. The general belief is that dragons don't exist. Most people are of the opinion dragons never existed.
Let's do a mental experiment. Assume for some time that dragons did exist. Imagine them, living in the land. Imagine your dragon in front of your eyes.
What is your dragon like? If you have a clear idea, you can send a description to me, I'm quite fond of dragons. But if I had to guess, I'd say the dragons of most people's imagination would share many common characteristics. Each dragon would be powerful. It would be fearsome. It would be majestic. It would project an aura of respect. Most probably, it would be intelligent and wise. And it would have a very strong will.
There are other features where they'd differ, of course. Some of your dragons would have wings and could fly, some could live on the ground or under water. Some legends say they breathed fire, while others disagree, saying they only had a very smelly breath. Many believe dragons could talk, others point out it would not be possible with the anatomy of their mounts. But these are unimportant details (at least for us, if you meet a dragon whose breath is smelling more than very strong cheese, it is indeed important at the moment for you).
And then you have the humans. Petty, weak creatures. They are of varying intelligence, but seldom wise. But we are very sure they do speak and don't breathe fire (at least without special equipment). Their only strength is there's a lot of them and they can get organized if they have to.
Usual situation could be we have a village a town. And some distance away a dragon lives in a cave. The dragon doesn't really bother the people in the village. It has no reason. They are little and not that much tasty to try and eat, provided they'd unite and try to defend. So the dragon does not attack them unless provoked somehow (let's say by stamping through its cave and trying to steal its possessions). Indeed. Look at how we humans treat ants. If they get into the pantry, we do something about them, but as long as they keep to themselves, we don't really care about them at all.
After some time, some people discover the dragon is wise and intelligent. If it is treated with proper respect and dignity (or, let's say, worship), they can come and communicate with it on some level. Maybe even come for an advice from time to time. People prosper from the advice, the dragon is happy for the company (or puts a „Don't disturb“ message on the cave door and people either learn to respect that or get eaten).
So, is there anybody who has a real problem with the dragon? No? Think again, of course there is. The king of the kingdom. Because, the dragon, while being mostly friendly, can not be controlled. It doesn't obey the king and even weakens his authority by providing good advice to the people. What if it advised the people not to obey him too. If the people joined forces against him, supported by the dragon, the king would be doomed. And it is not the question of whether the dragon wants to do that. It is the question that it could. That there's someone more powerful than the king. That's something the king can't stand (because he's used to being powerful).
So, what does the king do? He boots up propaganda that the dragon is actually evil. That it eats people. And that dragons are against the order of nature. And that dragons are risky for the survival of human race. And that whoever saves the kingdom from the claws of the dreadful dragon (read: kills it) shall get the princess as a wife and bunch of titles and lands as a bonus.
Because all the kings were the same, and the current politicians as well, dragons had two choices (because there were so many people who believed the kings). They could either extinct or hide well enough so nobody finds them. The result is the same. The dragons are forever lost for us, for few of our leaders feared something bigger than themselves. Don't let them do such thing to us again.