For the same reason Elfhelm swore in Westron when he tripped over Merry at the start of "The Ride of the Rohirrim" (unless Merry had quickly picked up enough colloquial Rohirric to understand '"dashed" tree roots...'and 'who left this blessed bag here are they trying to kill me?")...?
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“But Finrod walks with Finarfin his father beneath the trees in Eldamar.”
Christopher Tolkien, Requiescat in pace
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