That is a good question, i have been asking that, i have two guesses... first, just like Ruri's mother somehow rizzed a dragon (yes, she rizzed him, not the other way, that dragon is lucky she even payed attention to him), some dragons simply loved humanity enough to make a stand for them.
Second... despite some incredible abilities, dragons here do seem to have limits, the eletrict ability as example, it was said it is something that can take months even years to charge enough to really go wild, still strong even at a low output i have no doubt but the thing is... humans are smart enough and persistent enough to observe and learn, give it enough time and them developing a few ways to hunting dragons isnt far-fetched, even if the resources for that were really big.
And kind of a third that came to mind when I was writing... maybe fertility between dragons isn't particularly high, losing a few even if the others can retaliate, it puts strain on their numbers, so a treaty is better for them.
These are a few scenarios, and pretty superficially said too, these can have many more details going on, which could explain things, or there can be other explanations.
But i do think Shindou-sensei has thought of it, he just hadnt had the chance to explore that, there is a chance that he even just mentions it en passant in the story, as the focus may stay of Ruri just growing up, but i still think we will have more details... it seems there is a lot of surprises even between the ryus about how many abilities Ruri seems to have inherited, they may want to keep a close why on her too.