I cannot figure out why the US writes laws in terms of nominal value while maintaining an inflationary currency. The majority of lawmakers agree that a law should be x with respect to the current value of USD, why don't they also agree it should be x+inflation for a future value of USD? Maybe with a circuit breaker for hyperinflation. They don't have to rely on third party inflation numbers, because the Fed is a government agency.

Do some lawmakers only support a law because it will have a lower (min. wage) or higher (tax) real monetary significance in the future?

Probably, even if it is only accidentally. The Congressional Budget Office produces long-term estimates on things, and I'm guessing they aren't ignoring inflation. So, for example, a proposed law has $X ramification, according to the CBO, over the course of 10 years where X is an acceptable amount. If you were to adjust to make the law account for inflation, X would change, potentially into something not acceptable.