From this article:

Perhaps because of that, car-security experts say, people rarely pay them any mind, rendering them even less effective. Since blaring alarms usually mean someone accidentally bumped into a vehicle, or even just happened to play loud music down the street, an alarm rarely means an actual theft is taking place. Besides, if a thief really is trying to steal a vehicle, who wants to approach a potentially dangerous criminal? “You have a car thief attacking your car. You’re going to run out, and you’re going to do… what?” asked Reg Phillips, a vehicle-security expert who works with the International Association of Auto Theft Investigators. “What is in that car that’s worth getting hurt over?” (Of course, one could call the police instead.)

Moreover, a blaring alarm might scare off a first-time joyrider, but they’re a non-issue for most professional thieves, who can clip a few wires and silence an alarm with ease. Indeed, one 1997 analysis found that cars with alarms “show no overall reduction in theft losses.”

Worse, car alarms may be affecting the health of the people around them when they go off. A report from Transportation Alternatives, a bicycle-advocacy organization, estimated that New York’s car alarms lead to about $400 to $500 million per year in “public-health costs, lost productivity, decreased property value, and diminished quality of life.” An estimate from an organization whose stated goal is “to reclaim New York City's streets from the automobile” should be taken with a grain of salt, but the point still stands that car-alarm sounds are stress-inducing and sleep-interrupting.