Why exactly are custodial parents legally allowed to refuse to seek child support if the best interests of the CHILD should come first?

[Xen Master]

Why exactly are custodial parents legally allowed to refuse to seek child support if the best interests of the CHILD should come first?

Also, why exactly don't children have a legal recourse for such a move on the part of their custodial parents--for instance, by being allowed to sue their custodial parents for this (as in, for not giving them as good of a childhood as they could have by depriving them of additional child support) once these children themselves will become adults? For that matter, why exactly aren't children--once they will become adults--also sue their non-custodial parents for this if their non-custodial parents were actually in on this scam? Sure, forcing non-custodial parents to pay 18+ years' worth of retroactive child support would destroy their lives, but who the fuck really cares, right? The best interests of the CHILD come first!