Lol it's not flat earth. Who has the egg? Whose cell is the egg? The zygote is primarily made of the egg and the division and supplies for such are from the mother.
Again, cow's milk goes in your mouth. Whereas this is being synthesized in your uterus. It's different, no matter how much you want it to be the same. It's more akin to a tumor.
The cells come from the mother. They certainly don't come from the father or from nothingness. Ergo, from mom. That's how that works. Matter neither created nor destroyed, thermodynamics, etc