All because the manufacturers haven't worked out that salt water corrodes metal.
Seriously folks. PLASTIC ZIPPERS. They don't fail. Metal zippers all die within 1-3 years. No brainer. (note, some of these images are up to 10 yrs old, so perhaps some of the makers may have come around by now.... I'd like to see evidence of this...)

15+ years old, plastic zipper, still going.

BBong, not plastic

Not Plastic

Not plastic

Not Plastic
Been around the world a dozen times and still going, 15+years later. ALL PLASTIC.