Why was it a "sin" to eat an apple/fruit from the tree of knowledge?

Why was it a "sin" to eat an apple/fruit from the tree of knowledge?
Surely knowledge is a good thing.

Why would any parent (as "God" in Christianity is referred to Father) want their child to not have complete understanding or knowledge of the world?

Please do not answer with because "God" said so.
Why was it a "sin" to eat an apple/fruit from the tree of knowledge?
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