PCB return path

HI,

because in most cases it is. .. and one wants it this way.
For sure you can create a different path as return.

Basically you need to understand:
* current flows in a loop
* you have to differentiate between
+ low frequency signals (DC, only ohmic resistance counts) and
+ high frequency signals (HF, like the edge of a signal, where impedance counts)

Problems caused by the (return) path for DC: voltage drop, Ground bounce, heating
Problems caused by the (return) path for HF: characteristic impedance jumps, signal distortion, ringing, delay time (until stable), echoing, EMI, EMC...

But this is not a question that can be easily discussed as text.
--> You talk about PCB, so show an example of your PCB and tell which signal you want to talk about. Also we need to know the characteristics, voltage current, waveform of your signal.

Klaus

0V plane is a better representation than GND...

British cars once used a positive ground.

Ground is universally defined at zero volts as a reference, where or not it is floating from protective earth ground, or analog ground or digital ground.
Yet for each important voltage , there must be a 0V reference and that is called "ground" which is also by definition a return to the source regardless of polarity of the source. This means any measurement system may have different "grounds" dedicated for each measurement reference of 0V.

This must be defined by the user.

GROUND is ground, protective earth... GND and Ground are used but are technically incorrect, schematics really should use 0V's for the 0V planes and references...

Careful and deliberate planning of a ground return path will keep unwanted currents from forming in parts of your circuit where they should not be. Provide deliberate ground return vias and ground return paths for all of your signals—especially high-speed switching signals. source: https://www.allaboutcircuits.com/technical-articles/better-pcb-design-return-paths-impedance/

If you want to know more about how important is the GND in a PCB, you can read this too: https://www.nextpcb.com/blog/what-is-gnd-in-a-circuit