Like everything else floating in the ocean, where a bottle end up is the result of a complex interaction between waves, wind and currents.
Nothing makes things specifically go to shore, and we will never know how many of such messages are never found. Probably they are not nearly as common as cartoons make you think.
But if a bottle’s journey is at an end when it hits the shore, most random movements will sooner or later hit a shoreline somewhere and thus give the impression they drift there.
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