Yes, Blake and Adam do help their artists after the show. Perhaps the other coaches do too, who knows. The point is, why should we expect them to? They have their own careers and lives to live. We shouldn't be so selfish as to put the burden on them when the music industry isn't so easy to be in. And Blake is the only one who has it easy after the show because as been mentioned, he's solely in the country music and he has a huge impact there. Besides, country fans are loyal so not much help from Blake is actually needed. Also, even though the contestants are not superstars now, who knows what'll happen some time in the future? Not every superstar has a fairy tale story.
I agree with hincandenza on the fact that there will only be one winner so the best the other contestants could do is to get to the furthest they could in the show. The longer they stay, the more training, exposure, and connections they'd acquire. Experience is the best teacher after all.