It can seem dreadfully unfair. You struggle to buy the kids school shoes and your ex expects to see them when he/she chooses and pay nothing towards their keep. He/she may even demand that they arrive with a packed lunch !!!!!!!!!!!!!!!
Equally unfair is the dad (and it's usually the dad) who pays for everything so that his children don't suffer but is frequently let down by the mother cancelling contact visits, refusing ever to let him see them on Christmas Day, and treating his request to take them on holiday abroad in the same way an alien expedition to Mars would be greeted.
WHY SO UNFAIR?
The Court's concern is the child. The public policy is that it is the child's right to have a fulfilling relationship with both parents, no matter how bad or feckless those parents are. Provided, of course, that they are not at risk of harm.
The all paying dad would obtain more contact with his children if he went to Court, probably even alternate Christmas's, but he won't do it because he puts his children first.
The non paying dad (and it's usually the dad) does not put his children first. He may even quit his job to avoid a Child Support Agency assessment. I do not want to stand up for any of these people but the ones who show an interest in seeing their children are better, surely, than the ones who simply disappear?