Let's look at e = mc^2 , Albert Einstein's famous equation
The m actually is delta m, but delta m is m, m is mass. So, imagine an object, before it weighs 10kg, after being used to produce energy, it became 8kg, the 2 kg was mass too, the 2 kg now had became energy (conceptually speaking).
The equation tells us that such mass is energy, but through language we could get to the difference between mass and energy, when a mass is not convertable or when a mass is very stable relative to what's available in the surroundings, mass is mass and it's distinct from energy. When a mass is volatile like fuels, or if someone knows how to quickly convert dirt to something else, then the dirt or the mass is energy. For example 10kg of gasoline is 10kg of energy, but 100 kg of pavements are rarely energy, relatively speaking it's just mass.
So my argument is, you wouldn't conceptualize energy if there were no transformation of things. When we speak joule, we talk about conversion of things (e=mc^2 entails that the reference to "The power to convert things" is the very "Conversion of things" itself). Mass that doesn't convert might have the potential to convert, thereby energy, but... it's not really energy originally. By the virtue of language mass and energy are still distinct from each other, particularly, mass that are stable relative to the perceivable environment are not energy, yet. While mass that are convertible relative to what's available or what's going to happen in the near future, are then "energy".
But people perceive energy to be something other than that right? When we feel energy, we feel the key to unlock the conversion... those keys might not be the things that are being talked about in e = mc^2
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Edit: Ok so I think about this further, I noticed that my assumption was wrong. The e here is not all e ever, it's just mass is energy. So, not all energy manifests as mass, not all e that currently exists took the form of mass
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Check out this video: The Science Asylum's: Why Doesn't Light Have Mass?
So e = mc^2 + pc^2, not just e = mc^2
