Because an eV is a unit of energy (1 eV = 1.602e-19 Joules). It's defined as the kinetic energy of an electron that is accelerated by a potential difference of 1 Volt, nothing to do with its rest mass.

When rest mass is stated in energy units such as eV, they're calculating it using E=mc^2.

You can express rest energy in electron volt, a unit of energy. The energy an electron gains by being accelerated in an electric field of 1 Volt.

But since E=mc², you can say the mass is X eV/c² and then people become lazy and forget the c² or even define a new system of units in which c=1.

- "or even define a new system of units in which c=1"

https://en.wikipedia.org/wiki/Natural_units

More specifically it’s that you can express mass as energy, and that voltage is the energy per unit charge

Volts are energy per charge, right? So the cost of moving X charge from one place to another where the electric potential between them differs by voltage V, is X V?

Kind of like how lifting a mass involves increasing the g h , so costs energy m g h ?

Others have explained it already, but just to make this clear: The mass is not expressed in volts but in "electron volts" that is a different unit in the same sense as "watt hours" is a measure of energy and not time.

You can't express mass in volts. A volt is energy per unit of charge. To get energy, you need to multiply by a charge.

One Joule of energy is what you get when you move one Coulomb of charge across a 1V potential.

One electronVolt (eV) is the energy you get from moving one electron's worth of charge across 1 volt of potential.

It's an accident of what we chose to be a Joule of energy and what we chose to be a Coulomb of charge, so there should be no expectation that this would turn out to be the mass of an electron (when divided by the square of the speed of light, which is unstated because everyone knows E = mc^2).