Why don't they ask about my expenses?
A: The support amount is based on what you earn - not what you spend. For most of us, there is a limit to what we can earn, because of the company we work for, the value of the job(s) we do, or the number of hours in a day. We only have so much control over what we earn. BUT, we can control (or fail to control) what we spend. For example, you can buy an expensive new luxury car, and spend a lot. Or, you can buy a used economy car and spend very little.
If child support were based only on what was left over after the money was spent, there would not be much left in many, many families. Many families actually spend more than they earn, by using loans and credit. (Take a look at how an imaginary couple in New Jersey spends their money in the attached story; Tom and Judy were smart people).
Kids can't support themselves, and the law puts kids first. Therefore, child support is based on the total earnings of parents with only a few mandatory things, like taxes, deducted. It would never be fair to children to base support on what is left over after expenses.