This is what I have understood...
The problem is that as the output of the pickup increases, more wire is wound into the pickup thus increasing the impedance of the pickup.
High impedance (Hi-Z) devices tend to pickup ambient radiation noise easier than Lo-Z (This is why guitar signals pick up lot of noise when transmitted through a long cable unlike mic signals).
Hence, high output pickups tend to pick up more noise.
The alternate to this is what active pickups do - use a low output pickup and boost the signal using a pre-amp built into the guitar itself. Now you have a signal which has the same net output as a hi-output pickup but coming off a Lo-Z source. So this signal picks up less noise from surroundings and also is now less prone to picking up noise during transmission over long cables.
-- My current rig
Guitar: '98 MIJ Ibanez RG520 QS w EMG 81/60
Amp: Laney LC15R Tube, Fernandes 20W SS
Pedals: MI Audio Tube Zone Overdrive, Boss OD3, CS3, MT2, HM2, DS2, CE5, DD7, TU3, Beta Aivin DD2 Delay, Dunlop Crybaby 95Q, Stranger Vol Pedal, BBE SupaCharger
Others: Yamaha RBX 374 Bass, Yamaha MM-6 Synth, M-Audio Fast Track, Shure SM-57, Heet Sound E-Bow Plus
Wanted(pm if selling): EB VPJr/Boss FV500L,Ibanez w Original/Lo-Pro Edge