I think what usually gets mixed up is how the responsibility works, and biological children sit at the overlap.

The thing I most crucially remember about my son being born is that it felt downright easy to simply dive into all the things I would now be doing: because there was no one else. I either got it done or it didn't get done.

Someone else's kids on the other hand there is a choice: their parents.

It's not absolute IMO but you also see it echoed by working too: when it's your job, it's a lot easier to simply go "right I need to handle this" then when it's not.