Because airborne salt attacks the aluminium and copper in an outdoor coil, and how much of it reaches you depends mostly on how far back from the water you are.
Salt spray from breaking surf becomes an aerosol, gets carried inland on the prevailing easterly, and settles on whatever is outside. On an air conditioner that means the condenser coil: thin aluminium fins mechanically bonded to copper tubing, with a temperature difference across them and a fan pulling air through them all day.
The two metals in contact with salt water make a small battery, and the aluminium is the part that loses. Fins go chalky, then powdery, then they come away from the tube. Cabinet fasteners corrode. Electrical terminals corrode. None of it is dramatic on any one day and all of it is cumulative.
This is not a reason to panic or to buy anything in particular. It is a reason to know where you sit, to rinse the coil with fresh water on a schedule, and to read the coastal wording that sits in a good many equipment warranties before you rely on one.