Because it's programmed to do so?

Everything an AI does, no matter what, it will do it because of us. Because we, either intentionally or unintentionally coded/trained it to do it.

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If you are thinking of some kind of awakening where an AI suddenly gets it's own different goals, you don't know what you are talking about. (Wich is understable, but I would suggest you to read instead of writting about it)