They sell them because they're amazing. And that's the most logic you''ll get out of Eileen on the matter--not unlike the fact her conclusion of why squids are superior to sperm whales always comes down to 'have you seen squids?'

Her fingers lace slightly behind her back to help keep her posture perked as she bobs her head fairly enthusiastically. "We could probably build a huge one between three of us. And--" tacked on quickly "--it really won't interfere with my studies, milord."