Iconic seafarer beards became a trend to scare away pesky mermaids.
Mermaid, seeing a bald pirate: Zero threat…!
Mermaid, seeing a hairy, bearded pirate: If I get too close their face tentacles will eat me…!
Most mermaids only grow short beards as not to scare baby mermaids (longer beards look like the tentacled mouths of hungry squids and octopuses).
Pirates use this innate fear to their advantage to protect themselves from mermaids, hence the iconic beards and hat:
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