Phasma, Vader, Lando, Malak, and everyone else with a cape don't go running along the floor on all 6 limbs do they? Nor do they start spinning lightsabers 360 degrees uber-fast which would most likely shred the cape. Also, he always takes his cape off to fight.
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They gave Phasma a cape
and Vader
And Landos
And Malak. Excuse rejected.
- Grand Admiral Thrawn