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Toriyama's always said he's a big movie fag. Look at some of DBZ story arcs, they are all practically movie rip offs. When it came the moment to give Goku an origin story in the manga, Toriyama just gave him Superman's backstory. He even made it more evident with the release of Dragon Ball Minus.
Toriyama likes Superman, look at Dr. Slump, his other popular manga. There is a parody called Suppaman.
