That's because Goku has a one track mind.
The thing about Superman and why he's so timeless is that he's reinterpreted differently in all his appearances depending on the writer. The Golden Age version was an enforcer who doled out justice to those the system couldn't help. Silver Age writers alternated between him being an angel of mercy and a sociopathic dick to his friends when they schemed to find out his identity/try to marry him. Optimistic writers portray him as everything from a pacifist to a system rattler for justice. Cynical writers seem his as a tool of the powers that be, while the most cynical writer on Earth, Garth Ennis, views him as a symbol of all that's right about America and immigration, essentially casting aside old grudges and helping the people of his new land.
Goku? None of that. Eat, sleep, fight, repeat. Maybe conquer the streak somewhere in there. Luffy, for all his many faults, seems to have people around him to direct him. Goku doesn't nor does he want to interrupt his schedule. At least that's Super's version. GT made him childish but he was ultimately heroic and responsible. It's reminiscent of Homer Simpson's decay.