A very important factor here is the direction of the load versus the run of the jackline. So a line runs parallel to the boat, fore and aft. A falling load (e.g. me) slips off the rail and into the water. The force of my weight is pulling in the middle of the jackline. This means the line has to hold my weight times the shock load times a multiplier (200% at least) due to the vector force.
This having been said, I agree there is a very large safety factor. But allowing for wear & tear, UV degredation, shock factor, vector forces, I like the idea of a large safety factor.