Cuz they write the laws.
Duh.
You might not get it, but most drug tests are not required by law, but are policies or requirements by companies, organizations and other entities.
Unions
No foolin.
If you have a high security clearance you are subject to urinalysis, but everybody else is protected by the federal government employees Uniom.
Politicians might be another matter, but if they’re not participating in operation golden flow, they should be.