In other parts of the world (with more sense) such as Europe, Australia, we're conditioned to always using it due to strict enforcement and as such it's not such a novelty.
NHTSA provides substantial grant funding for traffic-safety enforcement, conditional on the department's enforcement statistics for a particular set of safety-oriented violations, especially seatbelts. The brass encouraged us to take a minimum number of enforcement actions to maximize the chances of having the grant expanded. (It paid a for substantial chunk of our salaries and offered regular 1 1/2 overtime.)
A more likely explanation is the severe underfunding and understaffing of prosecutor's offices and state court systems. Most prosecuting attorneys are responsible for carrying hundreds of cases at a time-- they have no choice but to prioritize, aggressively.
(I don't think it alters your point at all because you're still talking about constrained resources and a need to prioritize, but in traffic court in my village the PD is the prosecutor; my negotiation was with a police officer and he in turn represented the results of that negotiation to the judge. Same with everyone else's tickets. Only more serious stuff involved the town prosecutor - he was handling code violations, underage drinking, and a DUI.)
The town prosecutor was handling the only DUI that day. (A town PD officer handled the lesser offenses, including mine.)
Pretty much every traffic officer I know could find at least 5 reasons to pull over any car, no matter how scrupulous the driver.
Fun fact: At the police department, we actually had the seatbelt alarms for all of our cruisers disconnected. Being able to get quickly in and out without the seatbelts snagging on our gear was the priority at low speed. Most officers are trained to unbuckle and be ready to exit the vehicle before coming to a full stop (technically illegal.)