Why is preposition "in" wrong in #1 but it must be used in #2?
1- This is the place where I was born (in)”
2- This is the place where I come from.
Neither of them require "in", so I don't know why you think it must be used in #2. In 1. the "in" component is contributed by "where" together with its spatial location function.
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Neither of them require "in", so I don't know why you think it must be used in #2.
In 1. the "in" component is contributed by "where" together with its spatial location function. In other words, "where" already means "in some place".