i never watched too much of it but in legend of korra only korra and aang know air-bending
why dont they revive it instead of keeping it to themselves
theyre all selfish
Not true, in Legend of Korra Tenzen and his family also know air bending.
because that's not how bending works
it's never actually justified but either you have one or you don't
It is justified, in The Legend of Korra second season. Different people go to different lion turtles, receiving one of the four elements. Their descendants either posses no bending or the same type of bending as their ancestors.