This is sort of a stylized high-level description of how the U.S. government works, but it isn't really true. Law is often made by the courts (we call it precedent). Law is also made by the executive branch (by writing detailed regulations).
It does have to be justified based on existing law and this is important. But often, existing law only talks about broad principles or is contradictory, and judges are expected to fill in the details. In the case of the U.S. Constitution, these "details" can be really huge gaps.
