Because its the zero reference.
It is not logical to say that the potential of an earth-connected object is zero because the earth itself has a non-zero potential. When an object is connected to the earth, its potential will tend to equalize with the earth's potential. So, the potential of an earth-connected object is typically not zero.
The gravitational potential at Earth's surface is considered to be zero as it is the reference point from which gravitational potential energy is measured. Any object at Earth's surface has the potential to fall due to gravity, and this potential energy is typically defined as zero at Earth's surface for convenience in calculations.
Your question should read, 'Why does a neutral have zero potential?' 'Voltage' means 'potential difference', and you cannot have a potential difference at a single point. A neutral doesn't necessarily have zero potential although it is connected to earth (ground). This is because the potential of earth isn't literally zero; it's just considered to be zero, in the same way that sea level is considered to be zero in terms of height. Furthermore, there is often a voltage drop between the neutral and earth -in which case, the potential of the neutral can be several volts higher than the potential of earth.
It is logical to say that the potential of an earth-connected object is zero because the earth is a large reservoir of charge that can absorb or supply an infinite amount of charge. When an object is connected to the earth, the charges on the object will redistribute until the potential of the object is the same as that of the earth, which is considered zero.
Zero
Earth potential is considered as zero potential because it serves as a common reference point for electrical systems. Any voltage measurement is referenced to the earth potential, making it convenient to establish a standard of zero potential for electrical circuits and grounding systems. This helps in ensuring safety and proper functioning of electrical equipment.
Earth is considered to have zero electrical potential because it serves as a common reference point for measuring the electric potential of other objects or points in a system. Since Earth is so large and its charge is effectively neutralized by the presence of both positive and negative charges, it is commonly chosen as the reference point for zero potential in electrical circuits and calculations.
By general agreement, it is conventional to think of ground, or earth, as being at zero volts.
That depends on what reference level you have chosen. If the (arbitrary) reference level you chose is Earth's surface, then anything on the Earth's surface has zero potential energy. If you choose some higher reference level, an object on Earth's surface has a negative potential energy. If you choose a lower reference level, an object on Earth's surface would have a positive potential energy.
Zero. - But only because the Earth is the referance point against which everything else is measured.
It should be zero.
Choosing Earth as the zero potential in practice provides a consistent reference point for electrical systems, ensuring safety and standardization. It simplifies calculations and measurements because Earth is a readily available and widely accepted reference point. Additionally, using Earth as the zero potential helps to prevent electrical shock hazards by providing a path for excess current to safely dissipate.