You mean, in single coils?
High-output pickups commonly feature additional windings and/or a stronger magnet, to generate a
stronger output signal sent to the amp. The coil of wire tends to serve as an "antenna" that will pick
up EM noise, including the EM field generated by 50/60 Hz alternating current. Adding extra windings
in the name of higher output will make tend to make this "antenna" more sensitive.
There are of course, MANY ways to limit susceptibility to EM noise, but the real problem is that by
making the pickup "hotter", the highs and midrange are pronounced at the cost of very murky lows.
It's also obviously a lot easier to boost a mild signal in order to overdrive an amp when you want (via
stompboxes, etc), than to take a strong, dirt-tending signal and dial it down to clean intimacy.
JM2