Active VS. Passive
I quote from this link...
"There are three primary reasons to power a circuit: to boost the signal going through it, to lower the ouput impedance of the signal, and to isolate the signal from interference from other components. Lowering the output impedance increases the efficiency of the signal traveling along the cable. Poor efficiency can result in muddy tone, loss of highs, and low signal levels. Isolation can prevent "loading" (a drain of energy) caused by the instrument cable and other components. A classic example of this is with a two-pickup passive Jazz bass: when you set both pickups to equal levels, the overall volume of the bass drops a bit, and this is caused by the pickups loading each other down. Note that many "active" basses use a passive blend pot, so they can have the same problem of a volume drop at the center of the blend knob."
I'm sure your pickups can defy the laws of physics. but for everyone else, this is why we use on board preamps.