Why does E=mc^2?

His equation is correct, but the narrative is wrong. c2 is two perpendicular accelerations. The result of such, is a confined rotation.....angular momentum.

E = M c2. E is un-confined motion. M is confined motion. c2 is what confines it.
 
This equation is only valid for the energy levels of an existing particle. One can not manufacture a particle with this equation.

One can not make a particle out of EM radiation.

Man can not make matter. He can only disintegrate it.