I think in terms of context as you mentioned you don't have children nor want them..
Imagine whoever you are closest in the world to, someone where you feel their pain and try to help them whenever there is an issue, instinctively support them and would put yourself through pain to prevent them feeling pain themselves.
Then, times that by quite a lot, as the bond between parent and child (not all but a decent amount) is extremely strong.
Also imagine they are young, vulnerable, may not be able to say or explain if they come to harm if they are below a certain age, and that you are their entire world
Would you force them into a house, away from you, who loves them and for most people enjoys and cherishes the time with them, to stay with someone who doesn't want their company and may not look after them to a high standard. Not even thinking of neglect but pushing them to stay somewhere with a disinterested parent who sits them in front of the telly, feeds them rubbish food and sits and games themselves - when they could be spending quality time with a parent who actively teaches them, nurtures them, and enjoys doing that?
It's a burden on the single parent of course, and I have seen the burden and the stress first hand with a number of people, but the child is so well cared for, bonded and loved, and they would sacrifice everything in themselves for the happiness of their child