Because structure is a commonality across all drama. Despite what pedants think, nearly all Western narrative consists of a beginning, a middle and an end. Audiences enjoy seeing character growth. The fundamental cycles of the monomyth, Campbell aside, can be found in a plurality of stories and legends across cultures. Psychologically speaking, we like to have certain elements in our storytelling. Alexander Polti divided all possible works of fiction into 36 dramatic situations and when you look at it like that, everything is "derivative" of whatever came first, whether or not the author is even aware of prior works.

It has been argued that The Hunger Games is "derivative" of Battle Royale but this argument is largely made by people who want to hate on Suzanne Collins. The fact of the matter is, both stories deal with schoolchildren made to fight on television. Collins had a much different message to convey, however, and although the scenario is the same the denouement and plot are very different. Besides which, Suzanne Collins was unaware of Battle Royale until Hunger Games was finished, so if the definition of "derivative" includes derivation, Hunger Games ain't. Both stories owe an awful lot to Walter Moudy's "The Survivor" but dollars to donuts neither one of them had ever heard of it. As a part of the collective unconscious, maybe - but then, all three stories owe their existence to Fredric Brown's "Arena" which most people know from this:

Now - is Hunger Games derivative of Kirk v. Gorn? NO. But they definitely have elements in common.

Wanna see derivative? This is derivative: