Because my past self had done it for me,
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It's because there were people who did something for my past self that I exist now.
In order to affirm my past self, I can do it for others without charge.
The point here is that it's not for the purpose of profit.
However, the upside (potential benefits in logical space) is huge. As an answer to this, I think there are things like "humans created by something are Gift" in a broad sense.