The blood/urine/breath samples come AFTER the arrest. At least where I live, blood needs a warrant, which they aren’t getting without enough ‘evidence’ to convince a judge. Hence the roadside sobriety tests. They can take a breath sample because the law requires that you give one if you have a driver’s license (it’s part of all the legal paperwork you sign when you get one), but you can’t really force someone to give a breath sample because it’s not a simple thing.

The breathalyzer and blood are definitive tests, but they aren’t needed for the arrest or the conviction. A cop that I know said the best cases were the ones where he stood a driver in front of his car’s camera for about five seconds and you can see them visibly fall over or stumble while just trying to stand there. Criminal cases are always about convincing a jury, and that means that ‘evidence’ like a car swerving on the highway, the smell of alcoholic beverages, the field sobriety tests, the general appearance or manner of the driver, the statements made, etc., all matter.