It's a variable resistor connected between the null adjustment pins of the op-amp. This allows for an adjustment to null the op-amp offset due to bias currents (among other causes).
Probably the same reason many spell analog, analogue. It's Tradition! But I think a zig-zag line is easier to interpret as a resistor, as compared to a rectangular box, which could be anything, just as two parallel plates represent a capacitor, and a series of looped lines represents an inductor, better than a box or circle would.
The rectangular box was introduced because original drawing packages did not have the zig zag capability. In addition, the resistor value could be placed inside the rectangle. I changed from zig zag over 30 years ago for this very reason.