It's a glaring hole, but of course the real reason this doesn't happen is because if the Baku say no, they lose audience sympathy, and if they say yes, there's no movie. That's why the story of this movie is weak.
Dougherty: "Hey, can you move off this planet so we can research it and develop medicine that might help billions of people in the galaxy?"
Balu: "Hell no. Screw the rest of the galaxy. We have already lived here for 300 years and want to continue to live forever!"
Picard: "Anij is hot! Beverly who?"