There's an effect, I forget what it's called, where a deterrent is effective enough that the only time you see it activate is accidentally. I believe this may be the case for car alarms. It has a side effect of desensitization, but it is still effective. I doubt most thieves smash and grab without consideration of alarms on the grounds that probably nobody will care about the alarm.

Thieves want to steal without being noticed. The real threat of a giant siren bringing attention to them with a towel wrapped around their hand standing next to a shattered window deters a lot of opportunistic theft. It isn't quantifiable, but it is no doubt there.

Determined thieves of course will find a way, or find less secure targets. But most theft is opportunistic.