I like it tho I always wonder why manga never have the concept of coparenting

It’s because until April of this year coparenting wasn’t a legal option really, before, Japan mandated sole custody to one parent, which often completely severed the non-custodial parent's rights.
It was the concept of severed family name (clans in the old days) rights, so when you divorced the child would belong to the family/clan of the custodial parent’s, severing all interaction with the other.
That’s why in mangas and anime was so common to see children having to decide the parent they would live with, knowing that they will not see the other one again.