Alarm systems do work, check out this video as this would be crook is deterred away from breaking into a house because of an alarm system. This was filmed in broad daylight as well. Also, notice there was a way the homeowner left for the burglar to get in if the alarm had not gone off.
Notice the beeping of the alarm at the end. Even if it were not a real alarm system, the crook would not have known the difference. That’s why as a home owner, you have to find out what items deter the would be thieves and put those into effect.