Because it's plausible to believe Paarhanax may try to return things to the way they were prior to the Dragon Wars, albeit with some changes. You can ascribe any motive to that - from him being a card-carrying member of the "Rule the world club", to thinking that was the best way to keep the peace between dragons and men because that's the way it was until someone upset the apple cart.
He's had 5000 years to make power plays. Instead, he sat in seclusion, teaching pacifism. Hundreds of years went past when there were no dragon hunters around. Why make his move now? If you want to be paranoid, better watch out in case Lucan Valerius decides to stab you in the back for shaving some gold off his Golden Claw. He does comment on it being lighter than he remembers after all, and the Valeriuses tend to go on about nothing but the Claw...
You can ascribe anyone any motives you like, but there's very little in the game hinting at Unca Paarth wanting to reclaim his leadership position of the dragons. Instead, it comes of as you
wanting to ascribe them less-than-altruistic motives, for whatever reason.