Part of it is that traffic laws are written so that pretty much anyone can be pulled over for something at any time. Makes getting a warrant generally superfluous when someone steps into a vehicle.
I mean, sure, plenty of couriers aren't so bright and do things like smoke a blunt as they're going along. But an awful lot of these? Yeah, the stop has absolutely nothing to do with with traffic laws.
Why do you think so many transcripts in suppression hearings and criminal cases include police statements about how, when they approached a person/vehicle, the "corner of a baggie" was conveniently "sticking out" from something?
Answer: BS traffic offense used to pull over car. Car illegally searched. If no drugs were found they'd have sent the people on their way (what, they gonna sue for an illegal stop when no one was beaten to a pulp? Hah! You'd have shit luck even if you were beaten to a pulp thanks to immunities).
If drugs are found, they just make up some more BS that puts the drugs in the area of law known as "plain view". ie, the cop is allowed to stand outside your window because of the BS traffic stop, and anything in plain view is fair game. Naturally, anything not in plain view in reality becomes in plain view in the telling, if the search is ever challenged.
Meanwhile, the populace doesn't care. Shouting love of the constitution is a lot easier than letting people who really were a'criming go free, so amendments like the 4th tend to mean a whole lot more on paper than in practice. Anyone gonna care whether or not this particular stop and search was legit? Nope. She had 27 pounds of drugs. To many, the constitution only protects "law-abiding citizens". . . . which is the opposite of its point, but what can you do?