Becuse Jounin is the highest ranking level. The next step is Kage.
But not all Jonin are equal.
For example
If Asuma was set up to fight Darui, could Asuma win? I don't think so.
If it was Kurenai who was given the Land of Waves Mission and she had to fight Zabuza, could she win? No
If Yamato and Akatscuchi had a no holds bar throw-down, would Yamato win? I don't think so.
We already know Kabuto can take Shizune.
What if Anko was tasked with fighting Kurotsuchi?
Put the dad version of the Ino-Shika-Cho against Killer Bee (at Bee vs Taka fight) and it's a slaughter.
These are just the jonins of other villages vs the Leaf Jonin. You do see a disparity in their abilities. I didn't bring up any Akatsuki members, because they are so specialized and exceptional, but they would stomp the Leaf Jonins too. Add in people like the Gold and Silver Brothers and the 7 Swordsman of the Mist or the Jins killed by the Akatsuki, and the limitations of Leaf Jonin become more and more apparent.
Now, I'm not saying they all need to be distinct and powerful, but looking at the names of the Jonins we know, most are nothing to speak of. Better at beating up chunins and jonins weaker than themselves, than trading blows with the best from other villages.