Because I think it's bullshit (either superstition or a lie). "Might sometimes interfere in an undefined way with an unspecified device" is a non-falsifiable claim.

If somebody told me not to fart during takeoff and landing (because it might change trajectory of the plane — you never know…), then I'd still protest even though it _might_ be safer not to fart.

Phones are supposed to pass FCC EMI tests and I expect planes' electronics to be properly hardened/shielded from EMI even stronger than tiny consumer devices can produce.