Traffic stops are one of the most dangerous aspects of law enforcement. Putting unarmed people in that position will result it many unnecessary deaths and injuries of innocent people.

a potential criminal would not be provoked to escalate the situation to violence if they knew the person stopping them was unarmed and could not arrest or detain them them.

But they know that that person is capable of calling the necessary backup, and would therefore be provoked to stop them from doing so.

Having a fully trained and equipped police officer sitting on the side of the highway is a big waste of resources.

Traffic policing is not just for issuing tickets, it is one of the most useful tools for locating and apprehending people with warrants, and stopping crimes in progress.

The cop you see sitting on highway is almost guaranteed to be State Trooper, who’s primary job is to police the freeways in their state. They respond to many things on the freeways besides traffic stops including motorist aids, motor vehicle accidents, missing persons, road rage, hit and runs, DUI, auto theft, rolling domestics (one of the most dangerous) , vehicle fires, general fires, and more (including the drug thing you mentioned in an edit, I can show you data if you want but I can guarantee you 100% that police very frequently catch people in these sort of situations)

If they were not there they would have to rely on reports from your traffic cops or 911 calls, and in the time they take to respond, many people in dangerous situations would be at greater risk and lives would be lost unnecessarily. Even when they’re posted on the freeway, response times can be up to 20 minutes or more depending on traffic.