Legacy
I think it's rather obvious why it's UNLIKELY cockpit crew.
Let's do a quick pshycometric test:
Q: Who is MORE LIKELY to smuggle drugs?
a) Well paid proffessionals (doctors, lawyers,accountants, pilots ect) or
b) Low paid unproffessional workers (shop assistant, receptionist, cabin crew ect)
c) None of the above
Take your time before answering.