Capacitive proximity detectors rely on the fact that the capacitance is dependent on the dielectric of the capacitor.
Dry air has a dielectric constant close to 1.006, which the human body has a dielectric constant of about 80.
Thus, when a human enters the field of a capacitor, the capacitance will increase by up to 80 times the former value.
The electrical flux between the sensor wire and ground thus increased dramatically.