Why do you need to test a person's high? I don't care if a person has had too much to drink, too much to smoke, or taken to many prescription pills, if they are

Why do you need to test a person's high? I don't care if a person has had too much to drink, too much to smoke, or taken to many prescription pills, if they are driving poorly under the influence of drugs, they should get a DUI. Having a test just makes the prosecutor's job easier, but the officer could always arrest the people he suspects are too high to drive and get a warrant for a blood draw.