Yin Rentian could always play the role of stork and drop the baby at the doorstep of the Mo family but what then? Wouldn't his mother continue disrupting their lives?
This time around this child was lucky, but what about the next time? And the time next to it? He could not leave the child in the Mo house as he knew that there was no point. His mother would never let this child escape.
So he might as well raise this child for a while. Once his mother realises that the Mo family truly were not hiding the child and had no idea where he was, she was bound to give up and look for another source to create her humanoid heart.
His mother did not have the time to keep searching for the child, as long as she realised that she was fighting a losing battle she would give up—she was bound to give up.