Because, really, why the heck not?
Obviously, this is more interesting if you understand Japanese, but the refrain is in English, so there's that. The whole thing is rather charming.
These gents are actually comedians whose schtick is that they dress up as Dragon Ball characters. They've appeared on television, doing their DB bits (here, here and here).
ドラゴンボール芸人 ガチラップ Battleful Days [RYU@YouTube]
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