Why are urine tests so necessary when on methadone?

Other drugs. To see if you are taking the medicaton and not taking any other abusive medicatons with it.
Safety. Any patient on chronic opioids should have random urine tests. This is both a medical and legal issue and a national standard of care. Once a patient has been completely compliant for months/years then frequency can be decreased. If a patient has a bad outcome on these medications and the md didn't do proper surveillance for risky behaviors the md is likely to be a target of dea and lawyers.

Related Questions

Do I have to take 2 urine tests one is a methadone and the other a drug panel if the dr didn't discuss them to me?

Drug Testing: Pain. Pain management allows a physician to demand regular screening for drugs, especially those they are prescribing. If you do not do the drug screening the physician has a right to stop prescribing the medications you are on.