1. Uniforms can be a kink overall, more than just for roleplay. It can also be part of a perfect elegance package, but for professionals rather than just dressing up. 2. Flight attendants are overwhelmingly young, attractive, and female. The whole "mile high club" thing started out for a reason.
It's roughly the same reason "office lady" is a genre, though "flight attendant" is closer to the "secretary" kink, in that flight attendants and secretaries both have service jobs.
There's a decent amount of flight attendant porn out there that might work better for you in live action rather than anime-style pics.