Because they are universal. The pig tail on your factory wiring harness was not designed to be cut off and spliced to a universal sensor. The easy way around all that, and to ensure the job is done properly, is to buy an OEM sensor. Yes, it costs more, but you will have fewer headaches and hassles.
O'Reilly Auto Parts
Sounds like you have a three-wire sensor, and the black wire will be the signal. The two white wires are for the heater circuit, and can be connected to the vehicle's two heater wires without worrying about polarity. If there is a fourth wire (and there doesn't have to be) it will be gray. This is the ground. Most aftermarket O2 sensor manufacturers use this same black/white/gray protocol, but you didn't specify the particular brand you are having issues with.
Your customers will still have to determine the coloring pattern used by their OEM manufacturer, in order to match the functions to each wire on the universal sensor. Due to the wide range of applications covered, it is impractical to provide a color chart for every manufacturer.
JS Auto Supply