Because the adoption system would be way worse for the kid, and they kinda cared for her and felt responsible , at least Sentarou did.
then this manga wouldnt exist
What so they should just go and adopt every kid in the system then they literally have no proof at the start she even really is their kid like if some random person walked up and was like yea this ur kid now raise them, would you
It ain’t that deep
Just put the kid out for adoption. Who just lets some random lady force u to raise some random child you don’t know.