Why are child support laws so complex and unregulated throughout the states?

That is an easy one, to keep lawyers employed. Child care could be so simple:
Set childcare rates at a percentage of what an individual makes.
As the persons income goes up it can go up with it, if it goes down it adjusts to it. If they are unemployed a minimum level has to be set.

Next take out of their paycheck before they see it.
And lastly, give the poor CS payer a bit of tax break, not all on the one with the kids. This would simplify the system, unclog courts, keep the parasitic lawyers at a level of income they deserve over what they rip people off on.

No more going back to court, it is set until the child reaches an age determined by law to when it gets cutoff. If you are millionaire it goes to the same percentage as if you were a street sweeper. Equal in the eyes of the law with an equivalent pain point for all.

Child support should be a flat rate per child. Anything more than the minimum is actually spouse support

Your comment seems to conflict with itself. How can something be covered by complex laws, while at the same time being unregulated?

I think they're complex because trying to properly subdivide the parenting responsibility (and the resulting financial support) through the legal infrastructure is difficult to succinctly wrap up into simple legislation. Plus, since it's governed at the state level, you will have lots of variance across the states.